Its actually very simple. Most of the time, all I do is play a major (or minor) chord related to the melody note. So If I'm in Ab, and i'm holding an Ab up top, more times than not, I'll be supporting that with an Eb and a C, or something else in that scale, that fits.
Maybe thats the wrong way. I dont know. I never really studied chord structure or whatever yall call it. And I dont think about that kinda stuff when I play