Tuck the fleece tube inside the print tube, and sew around the already-sewn opening, to prevent shifting.
Carefully insert the foam between the layers.
Tuck the raw edges in, and pin.
Sew around this edge, careful to keep all the raw edges folded in.
I then sew around both edges a few inches in to catch the foam and prevent shifting. Presto!
If you're using the boning instead of the foam, I sew it to the wrong side of the print fabric before starting step 1.