Step 1: Don’t measure the actual pillow. Instead, measure the pillow cover that’s already on it.
Step 2: Decide that the cover on it has too much overlap and subtract a few inches from the length of the back pieces.
Step 3: This is the crucial step. Do not think even once about seam allowances. Then cut your fabric.
This would be what I did this weekend. (Can you tell I’m a very amateur sewer?) I hemmed the pieces that overlap, pinned all my pieces together and realized they barely touched. So I repinned it so that they overlapped about half an inch, but that took away from the length of the pillow. I thought it was a total failure, but figured I’d sew it up and try it anyway. Miraculously, it fit! It made for a very firm, plump pillow :).
It peeks out a bit in the back, but who sees the back anyway?
This was my other accomplishment. Much less stressful 🙂