Assuming that you have pieced enough blocks for the pillow top (not bottom), and that the other side will be palin old muslin
Sew the muslin to the back of the pieced blocks - like you have sandwiched a quilt that has no batting
Stitch in the ditch to secure muslin and blocks,
Stitch 1/4" around edges make sure all the seams of the blocks don't come undone.
Then construct the pillow as you usually would.