Diatonic Harmony... try this

You need to look at the chords, and figure out which scale they fit with. If you had a Bm, D, E progression you'd be playing a II, IV, V in A - and B Dorian would work well as it's the 2nd mode of A Major.

Also, as it's not in the other post, 7th chords follow this pattern (G for this example):

Gmaj7, Am7, Bm7, Cmaj7, D7, Em7, F#m7b5 and back to Gmaj7.

Once you get familiar with this stuff, you'll also need to know about common chord substitutions. In the example you give for a chord progression, the chords are not all the same key, so you'd need to be a little more creative...

The natural minor scale comes from the 6th degree of the major scale...