First choose a pillow. Any pillow will do. For this tutorial, I used a 12 x12 pillow.
Then cut 3 pieces:
Front piece, 12 x12 inches.
Yes, cut the fabric the same size as the pillow, this makes the finished pillow look fuller.
Then cut the two back pieces. I used a 6 x 12 inch piece + a 9 x 12 inch piece.
Now we will finish the back pieces.
Start by ironing one side of each piece.
Press raw edge down 1/2 inch.
Fold + press another 1/2 inch.
Using a 1/4 inch stitch, hem the back pieces, only one side.
Now assemble your pieces.
Lay your front piece down, face up.
Lay one of your back pieces on top of your front piece, right sides together.
Lay down the other back piece. Line up the corners, back pieces should overlap. Pin the edges.
Using a 1/2 inch stitch, sew along all 4 sides, backstitching in the corners.
Trim your corners.
Using a bone folder or chopstick or anything somewhat sharp, push out your corners.
Turn the whole piece