CHURT
THE 6-STRINGED MENACE! THE BASICS OF GUITAR NOTATION
A menace might be a little too harsh, but it has six strings. Well, at least for the sake of this article lets stay with six — a standard build six-string guitar with a standard EADGBE tuning.
The standard notation is already well represented in the classical guitar world. In this article, I wish to go into details of jazz, rock, and pop guitar writing, as well as how to write well-defined notation that is easily readable and consistent throughout your work.
THE INSTRUMENT
The guitar is a transposing instrument. Written guitar music sounds an octave lower.
Guitar range:

THE NOTATION
Legend
The notation discussed isn't set in stone. Many different sources address it differently. With this in mind, I advise you to include a notation legend to all guitar sheet music. I usually append it above the title. It makes it visible and very hard to ignore.

Chords
As with many, when faced with notating for guitar, I failed miserably. The music was sent back to me with a single word instruction: "Simplify!".

The issue; Written out chords! They are daunting and face the guitarist with many problems from fingerings to voice leading. I had to dive into an exploration of already published materials and ask guitarists for some much-needed advice. Thus the following solutions came to fruition:
Slash notation
The first most frequently used notation is slash notation. You can use it to define rhythms, rhythmical values, and accompaniment patterns. Its primarily used to map the music in the least constrictive way. It gives guitarists the freedom to choose their chord progressions based on chord symbols provided.

NOTE: You can choose to define the rhythms of the chords with the slash notation or choose to use the slashes only to indicate the chord progression movement.
SIBELIUS: use the slash noteheads by either going through Ribbon>Notations>Notehead or use keyboard shortcuts shift+alt(option)+ 3 or 4 (depends, if you want the rhythmic definitions or not ).
Top-line notation
When the slash notation isn't enough for the needs of the composition or arrangement, you have two options. Either you notate the correct voicing, or you use the top-line notation.
In top-line notation, you write out the top note of the chord and indicate it with a prolonged stem extending below the note head. This notation restricts the guitarist substantially more than slash notation but still gives enough freedom for effective voice leading.

NOTE: With this notation, I intentionally ignore the rule of inverted note stems and keep all stems upward, so it is less confusing to the sight-readers.
SIBELIUS: you'll have to create new noteheads to use the »top-line« notation. For that, go to Notehead menu via Ribbon as described in previous Sibelius note, select standard noteheads (0) and click on button new. In paragraphs »stem up« and »stem down«fill negative values into »shorten stem by:« section. I go with -3. Test what looks best for you.

TIP: Some guitarists presume that you want »top line« notation also when there are chords added above a written out melody line. You can avoid this by not writing chord symbols and adding a chord symbol N.C. (no chord) at the beginning of the phrase.
Standard Notation
If all else fails, you can resort to writing out a full voicing. However, my sound advice would be to reserve this only for the chords that are specific and play an integral part in the composition. You also need to be entirely sure that the voicing of the chord you wrote out is correct and playable.

NOTE: I always add a chord symbol above the written-out chord to make it easier for a sight-reading guitarist.
TIP: I can't afford to carry a guitar around to check all the voicings I write down. Instead, I use the many books of guitar chords that exist in electronic forms for convenience sake.
Multi-voice melody lines
When writing multi-voice melody lines, you need to be aware of the following:
- spacing between fingers on the frets,
- when and where will the guitarist change the position of the fingers on the strings.

HINT: If you are not very familiar with the guitar, I strongly advise you to avoid writing of counterpoint melodies and only stick to parallel melody lines.
Single Melody Lines
When it comes to single melody lines, the writing is straightforward.
Make sure to add an "N.C." symbol above the line to eliminate the possible mixup with top-line notation.
Other Effects & Techniques
Another topic to touch is the writing of different guitar effects and techniques. Over the years guitarists developed a new way of writing sheet music called tablatures. With them, they also developed a comprehensive system of marking of effects and techniques. The system of markings exists in standardized notation as well. However, composers like to mix the tablature notation markings with the standardized notation. I avoid this as much as possible and suggest the same to you.
TIP: Avoid the confusion by establishing your guitar legend. Print it with every guitar part you make.
Hammer-On & Pull-Off
While many like to indicate Hammer-On technique with H and Pull-Off with P, I avoid that as the information on the sheet music becomes too cluttered too fast. In general Hammer-On technique is used when a melody line moves upwards and Pull-Off technique when the melody line moves downwards. A simple slur line should do the trick.

Slides
A simple glissando line will most effectively indicate a slide.

Bends
To indicate a bend, you need to use a particular sharp-angled "guitar bend" slur line when connecting the notes.
SIBELIUS: The "guitar bend" slur line exists under Line submenu. The keyboard shortcut for the slur line is J.
For a quick bend, you notate the landing note. Before it, add an "acciaccatura" grace note that represents the fretted note and connect them with the "guitar bend" slur line.

For a bend of longer durations (Ex. 13: a), you can connect any other two notes of the desired length with the "guitar bend" slur line.
For a bend and release (Ex. 13: b), you connect the before mentioned quick or longer bend with the next note.
A pre-bend notation (Ex. 13: c) consists of a bracketed grace notehead connected to the landing note by a "guitar bend" slur line.

NOTE: The most known types of bends are full and half-tone bends. Occasionally you can stumble upon a quarter-tone bend or a tone and a half bend. When dealing with the latter two, I recommend speaking with a guitarist prior notation attempts.
Tremolo Arm
Commonly known as the whammy bar, is used to produce a wide variety of effects. To indicate the use of the bar, use the indication "/w bar" followed by a line, indicating the length of the usage.
Vibrato
You can use a wide vibrato line (Ex. 14: a) or a standard vibrato line (Ex. 14: b) with the /w bar indication.
SIBELIUS: Both lines are in the lines menu under "Ribbon>Notations>Lines. You can use a keyboard shortcut L.
Bar Scoops & Bar Dips
The symbol for a bar scoop resembles an inverted "guitar bend" slur (Ex. 14: c).
The symbol for bar dip is a slightly wider up-bow symbol (Ex. 14: d).
SIBELIUS: Use the symbol from the symbol sub-menu. You can access it from the Ribbon>Notations>Symbols or by keyboard shortcut Z.

Harmonics
There are many types of harmonics produced on guitar. They subdivide into two main groups: natural harmonics and artificial harmonics.
Till not so long ago I've used the symbol »◦« on top of the fretted note to indicate the harmonics. It was often missed or misunderstood as to what I wish from the guitarist. After much discussion with guitarists and upon further studies of guitar materials, I came to two possibilities that are clear enough for the performer.
Natural Harmonics
With natural harmonics, you'll have to know how high they sound.
Fret Sounding related to the open string
12 Octave higher
7 or 19 Octave + perfect 5th higher
5 or 24 2 octaves higher
4/9/16 2 octaves + major 3rd higher
3,2 2 octaves + perfect 5th higher
2,7 2 octaves + minor 7th higher
2,2 3 octaves higher
2 3 octaves + major 2nd higher
1,8 3 octaves + major 3rd higher
NOTE: The smaller the space between breakpoints the harder it is to get the intonation and sound of the harmonics right.

The notation for this is straightforward. You always notate the sounding pitch an octave or more below(if needed) and indicate the natural harmonic with marking H + (Fret), where H indicates that it is a natural harmonic, (Fret) is the number of the fret played to produce the harmonics.
Artificial Harmonics
When it comes to artificial harmonics, we also use a diamond-shaped notehead. However, we mark the fretted note, not the sounding note. That should be a good enough indication for the guitarist.

NOTE: There are a few types of artificial harmonics that guitarists use: pinch harmonics, tap harmonics, touch harmonics... If you know what harmonic should the guitarist use, make a marking above the diamond-shaped note (P. H. for pinch harmonics, T.H. for tap,...)
Mutes
There are two main types of mutes: palm mute and muffled strings.
The palm mute produces a defined pitch. For palm mute we use a P.M. (palm mute) with a line indicating the duration of the palm mute.

Muffled strings produce no defined pitch, rather a percussive sound to them. For muffled strings we change the noteheads from standard noteheads to "x" noteheads.

Other effects
Whenever you are writing for electric guitar, you'll face additional guitar effects. Be it distortion, chorus, flanger, ... You'll have to indicate them in sheet music.
If you wish for the guitar to play without any added effects, write "clean" at the section you wish it to be clean, at every other point indicate the effect you wish.
TIP: Simple notes are the best in this case. There are many types of effects producing the same result but have different specifications. When you are searching for a specific sound on the guitar, discuss it with the guitarist.
Conclusion
Guitar notation is a subject many avoid, as it is not as simple to write for it in detail as it is to overcomplicate the matter and ultimately make the sheet music a reading nightmare for a guitarist. Therefore keep it simple and always revise the sheet music in question.
TIP: I like to overcomplicate at the beginning. With the first revision of the sheet music, I usually get rid of about half of the information that I wrote down. Keep it simple and always have a sight-reading guitarist in mind.
I hope I helped you clarify how to address the many expressions of the modern guitar performance within the standard notation.
Please feel free to leave any questions or remarks in the comment section below.