I normally do arrangements for two to four guitars, but the method is usually the same and works OK for me. May be a bit clunky for others, though.
Anyway, here are the first few bars of Schindler's List, which I'm currently working on; I'll probably end up doing a solo and duet version.
(Note to mods - I'm sure this qualifies as fair use.)
1. Get the piano score into the computer (I use Noteworthy Composer)
2. Put into the treble clef
3. Experiment to find a suitable key
4. See what works on the guitar and decide what to leave in and what to leave out (that's the tricky bit). Currently, it's a bit 'full' but perfectly playable - as the piece progresses it might be necessary to simplify it a bit to retain the overall characer of the piece.
Of course, there's actually quite a lot more to it, as Pogmoor has indicated, but these are essentially the steps I use. Hope it's of some help.
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Lester Backshall, Guitar Maker - Aylesbury UK