Step 1: What You'll Need
- Kitchen shears. Sharp ones. Or an electric carving knife, saw, or some other cutting device.
- A staple gun or upholstery nails.
- Upholstery fabric. Goodwill and Craigslist are your friends here. Depending on the size of your window, a duvet cover could work. I went to Urban Burp in San Francisco, which was my favorite part of the project -- a cool store full of vintage textiles.
If you're thrifty, you can do this all for under $50. I went a little crazy by buying some really nice vintage fabric, so I came in around $100 all told (I scrounged almost everything, but paid $80 for fabric and $20 for plywood).
Step 2: Measure the window and cut the mattress
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.
Step 3: Make the diagonal cuts
The foam will have some straight edges at the ends if it's not wide enough--use the extra foam to cut some little triangles for each end so the piece goes from 72" wide to 90".
Saw your plywood to match. Then lay the mattress down on the plywood, and make sure everything lines up nice.
Step 4: Upholster that bad boy
I tried it out and decided it wasn't quite squishy enough, so I lined it with an old fleece blanket I had lying around.
There's a good YouTube video by ReadyMade that shows how to upholster a dining chair--that was my inspiration. It will explain better than I can.
Ta-da! Lay your new custom couch in the window and enjoy. Now make some pillows.