For the 1-Up Mushroom:
(36) 1.5" x 1.5" green squares (Kona Grass Green)
(38) 1.5" x 1.5" white squares (Kona White)
(24) 1.5" x 1.5" flesh-colored squares (Kona Flesh Pink)
(68) 1.5" x 1.5" black squares (Kona Black)
For the Background (Kona Medium Gray):
(2) 20.5" x 3.5" strips
(2) 6.5" x 1.5" strips
(2) 7.5" x 1.5" strips
(2) 5.5" x 2.5" strips
(4) 4.5" x 1.5" strips
(2) 3.5" x 7.5" strips
For the Lining (Kona Medium Gray):
(1) 20.5" x 20.5" piece
(note: this piece isn't a necessity, but I like to add it to the back of the mushroom panel to cover up all the raw seams on the inside)
For the Back (It's an envelope enclosure. Kona Medium Gray):
(1) 12" x 20.5" piece
(1) 18" x 20.5" piece
I wanted the background to be rather solid and have as little pieceing as possible, so sometimes I assembled a single row at a time and then attached the background strips to each end, and sometimes I assembled rows in groups and then attached the background pieces. For instance, rows 7-13 were assembled (postage-stamp style), and then I attached the background strips to the sides once that big hunk of squares was completed. Rows 15-16 were done the same way. I tried to put brackets on the right-hand side of the diagram so you can see how the rows/background pieces are grouped.
Here's a close-up of my pillow so you can get a visual:
For the back: Grab one of your back pieces, measure 1" down from the top edge and mark a line. Fold the fabric down so that the crease is on the 1" mark, press, and then fold down once more (another inch). Top stitch along the top and bottom edges of the fold. Repeat for the other back piece.
Layer as follows: Mushroom panel (right side up), Lining piece, and the Back envelope pieces (wrong side up). You'll need to overlap the back pieces in order to get them to line up with the rest of the pillow. Sew a 1/4" seam all the way around the pillow. There's no need to leave an opening, because you'll turn the pillow right side out through the opening in the back.
I intended for this post to be brief & basic rather than a full-fledged tutorial, but if anything's unclear, let me know in the comments and I'll try and draw and extra diagram or two.