Once you’ve got some simple progressions, you can then expand on what you’ve created to come up with progressions that are more creative and unpredictable. For example, you might take a simple progression like I-ii-V-I (C-Dm-G-C) and do any of the following:
Add a lowered-VII-chord, or change the V-chord to a lowered-VII-chord: C-Dm-G-Bb-C, or C-Dm-Bb-C
Add a secondary dominant chord. A secondary dominant takes a good deal of explanation, but to simplify it, simply take a chord that is normally minor, then switch it to major: C-D-G-C.
Change a ii-chord to a lowered-III (major) chord: C-Eb-F-G.
Change any chord to the version of that chord you’d find in a minor key. (If your chosen key is minor, change chords to the version you’d find in a major key): C-F-Fm-G-C