I usually start an arrangement with copying the melody and chords (changes) into Musescore from a Real/Fake Book.
Next I use the Transpose function to find a key that's most suitable for the guitar and the melody's range. Don't neglect the usefulness of a capo, e.g. if the original key is F, try an arrangement in E and play it with a capo on the 1st fret.
Once I determined the most suitable key, I start developing the bass line (I personally don't like the "chord melody" style where a block chord is played with every melody note), then I fill in the middle voices. Experiment with chord inversions to avoid duplicating notes, e.g. when the melody note is the root note of the chord. This often leads to an improved bass line. Also feel free to change the rhythm of e.g. the melody to give it more of a Jazz feel.
If you don't have a written source, the approach is more or less the same, i.e. find the key, write out the melody, then listen to the bass line to find the harmonies. With a bit of practice, you will start recognizing chords by their sound. A bit of theory knowledge helps as well, as many Jazz standards use e.g. I-VI-II-V and II-V patterns in different keys throughout.
I have lots of arrangements of copyrighted jazz standards available for sale on Sheet Music Plus, so in case you are interested in trying some of them search for Peter Miklitz there.
Good luck with your own experiments,
Dringt durch des Aberglaubens Nacht, die Euch zu finstern Köpfen macht. Christian Fürchtegott Gellert (1715 - 1769)