Step 2: Measure the window and cut the mattress
First, you'll need to know the measurements of your window. Bay windows are trapezoids (or rhombi? oh middle school geometry, where have you gone?). But I just use a rectangular measurement for the first cut of the mattress. Measure the widest part of the window and the deepest. Odds are your mattress isn't wide enough, but we'll worry about that later. My window is 90" wide by 31" deep. My mattress was 72" long. So I cut to 32"x72" (the extra inch of depth for a little overhang on the final product.)
Take the cover off your mattress to expose the foam core. IKEA Sultan mattresses have a grid scored into the top half of the foam. This turns out be a fantastic guide for the first cut. There are a number of implements you can use to cut (an electric carving knife, a saw, etc.) but I went with my trusty sharp kitchen shears. You'll notice it came out pretty ragged--look for something better to cut with if you're a perfectionist/pressed for time.
Save the top piece! You can use it to make cushions later.