Don't try to find a "fastest possible way". In any case it won't be fast and easy. You're asking about eating an elephant - the only way to do it is: one piece at a time. Learning to sight-read a piano piece and convert it to a working guitar arrangement on the fly does not happen overnight.
You need to split the task to individual skills and develop each of them separately. I can only do this by ear, not by sight-reading sheet music, but I suppose the things you need are the same when doing it by reading. The skills you're looking for are:
- Recognize chord changes. There may be lots of notes but the chord doesn't necessarily change.
- Classify each chord: is it a major or minor, sus4 or dim etc. and what degree. What is its function: tonic, subdominant or dominant of the key or its relative key? Maybe something like a secondary dominant? A short detour to a different key, borrowing chords from there? So, this gives you essentially the root and the third of the chord.
- Recognize bass note movements. The bass isn't necessarily the chord's root note.
- Recognize sevenths and other important tensions (and know what is "important" - you'll be simplifying things and selecting more important bits)
- Recognize the melody note - this is usually the topmost note.
- For each melody note, classify it as either a "chord tone" i.e. part of the chord, or a "passing tone"
- Know which chord changes are big and important, and which ones are more like "passing chords" that don't add to the whole significantly enough to warrant finding a whole new hand position and fingering. You're going to have to make compromises. Can you make easier reductions of chords, substitute overly dense chord changes with e.g. I / IV / V?
Can you do all of the above already? If not, start practicing. They are separate skills or skill steps, and all of them are required for what you want to do.
Let's assume you can do all of the above. When making a guitar arrangement, for each chord change step you collect the following pieces of information
- major, minor or sus4 chord, (or dim) and what degree, i.e. root and third
- bass note
- melody note
- possible sevenths (or other important characteristic tensions like b9 - if it's important for that piece)
Put these four bits of information together: select a chord shape from your palette of chord shapes such that you can have the melody note as the highest and the bass note as the lowest note. Then make sure you play the bass note and the third of the chord somewhere, adding notes to the fingering. Balance the chord tones so that if possible, you don't double any of them. For example if the melody note is the third and the bass note is the fifth of the chord in question, then you play the root and/or seventh of the chord with the strings between the lowest and highest notes. You also try to avoid too close voicings especially if they're low notes. And do all of these selections and trade-offs so that you can fluently move from one chord to another.
And of course, you'll have to be able to play the passing tones in the melody. This is done by either picking notes you're already fingering with your left hand, or hammer-ons/pull-offs, or by sliding a finger or even your left hand.
Sometimes you might want to change the arrangement to be able to play it. For example, reverse the direction of the bass movement, or substitute a chord with something else that performs a similar function. Or maybe even select a different function if it works with the melody and you can't find a way to play the original one comfortably. Sometimes, just one harmony note in addition to the melody note is enough for a passage. Sometimes you might only play a single-note melody line ... it can make a dramatic effect, and as an added bonus you can avoid difficult fingerings. :) Sometimes it might be easier to play a dim7 (rooted on the third, seventh or "sharp 1") instead of a plain dominant 7th, it does the job and may even sound nicer. It's your arrangement, so you'll be making artistic choices, not only technical ones. Nobody has to know if your choice was more technical than artistic. ;)
There's no magic trick that makes you able to instantly throw in chord inversions and fingerings in all situations like a virtuoso. It's a long journey and you'll have to do lots of hard work.
So: start acquiring the necessary skills. While practicing and learning, and when you aren't still able to put it all together, if you can at least play the chords and bass lines so that you or someone else can sing along with it, it will hopefully be satisfying enough to keep you going and motivated. Good luck!
