Writing a few songs right now and this form occurs in the verse of one of them. It is an 8 bar harmonic structure that could be labeled as such: a,b,c,c a,b,d,c. What this means is whatever chord u play for the 1st bar you should also play for the 5th bar. Similarly, whatever chord occurs in the second bar gets played again in the 6th bar. For the 3rd and 4th bars u want a single chord to last for both bars. During the 7th bar u should play a chord that has not yet been heard within the progression. And finally, in the 8th bar the chord that filled bars 3 and 4 should return. Here are a few examples of what this structure looks like in practice:
| I | V | IV | IV | I | V | bIII | IV
| V | IV | I | I | V | IV | ii | I |
| I | bVII | IV | IV | I | bVII | bIII | IV |
This structure is very effective for verse sections, but u can try it in other song sections if u prefer. So, give it a go and see where it leads u.