Stripe Cutting, second step: Change to cut (12) 3″ x 21″ strips, not 3-1/2″ x 21″ strips.
Piecing the Blocks, step 1: Use the 3″ x 21″ stripe strips to make the strip sets.
If you have already made your strip sets using 3-1/2″ x 21″ strips and cut your segments, trim 1/2″ off each stripe end of the segments to make them 3-1/2″ x 6-1/2″ with the dark texture strip centered.