I cast on 40 stitches (20 for each needle) using Judy’s magic cast-on. After the first round I increased 2 stitches on the back and on the front using la-linc and la-rinc.
After a few rounds I switched from 3,5 mm to 3,25 and knitted real tight because otherwise the cover would be too wide. I could also have cast on 2 stitches less.

When my cover was long enough I cast off half of the stitches and continued on the other half. When my flap was long enough I started decreasing every right row on the left and right with ssk and k2tog. I added a crocheted loop for a button to serve as a closure.