okay once you get the guitar part, you just need to work with how it sounds. a major part is just by ear.
for piano, you can play the same chords as the guitar but in different transpositions. you can also do some harmonizing on the piano too... its a very versitile instrument for rythems.
for bass, you just play the single note root of the chords. thats the easiest way.
drums (not my thing) just find a beat and stick with it i guess. haha
just think about what sounds good. if it doesnt sound right together, something went wrong and you should figure it out.
and theres nothing wrong with a one man band! ive had a one man project for a long time now. :p