In Lesson 6 you were introduced to the basic form of 12 Bar Blues that used the I, IV and V7 chords in the following pattern.
There are many variations of the Blues progression. E.g., the following Blues uses seventh chords instead of major chords, a IV chord in the second bar instead of a I chord, and a ninth chord instead of the V7 chord. Experiment with your own Blues variations, substituting different chords or chord extensions.