The idea is that behind any harmonization of a melody, by a chord progression in a scale and mode of it, there is an alternative harmonization with only 3 (1,4,5), only 2 (1,2 or 4,5 or 1-4 or 1-5 or 7d-1) or only one chord (1) but which must be neither major nor minor but a neutral power chord P (tonal-dominant-tonal or 1-5-8)
This harmonization is out of the usual style of guitar playing, but still it exists in arrangements where bass, or continuous bass, is engaged.
So the way to find this continuous bass isokratic (drone) line of only a few notes (at most 3-4 notes) continuous or drone bass is to use notes only at the 1st octave of the piano, as if whole chords, and fit it to the melody, changing it by intervals to the 2nd (rarely 3rds) to find the other note.
Usually the odd-numbered notes of the mode will give the first note, and all the even-numbered notes of the mode will give the second note. The changes will be when the melody uses more of the odd-numbered notes or more of the even-numbered notes.
It is obvious that if the player knows the usual chord progression, then he will simply play one note from each chord as the chords changes to produce a countermelody or bass line. This would be a method of giving many more notes and changes compared to the previous. It will also sound better fitting, but what it takes is that the chord progression must be known.
