I tell you what, first find the bass line, that is most important, I have to know what notes absolutly need to be there, then what ever free space I have, I generally like to play around the scale that song is played in, as long as you come back to the root you can basically play anything in that scale for instance if you are playing something in F then you can play the F,G,A,Bb,C,D,E, but be sure to resolve on that F and it should sound pretty tight. Peace!