Step 1. Start with corner and the right side up. You will need to calculate the depth of the corner. This 1/2 of the depth of the cushion + 1/2" seam allowance.
Step 2. Bring the two cut edges together and press. Now, measure along the cut the depth of the corner and pin. Clip 1/4" into the seam allowance at the pin.
Step 3. Fold at the pin or clip along the cut edge and press. Mark the corner on fold with a pin.
Step 4. Open up the fabric with the right side up.