In San Francisco, bay windows are everywhere. While many are floor to ceiling, mine have a built in wooden seat about 2' off the ground. They've become a real dead space in my apartment, and I decided to solve the problem by turning my bay window into a comfy couch with a custom cushion.

Step 1: What You'll Need

- An old mattress. I used an IKEA Sultan mattress (full size). Foam core mattress seems like the easiest way to go.

- Plywood.

- 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

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.

Step 3: Make the Diagonal Cuts

After you make the first cut, calculate the diagonal lengths for your window. For this, I had to call in some mathematical help. But be warned, as carefully as you calculate your cuts, they will inevitably be a little off. So be precise, then once it's all cut out throw it in the window for a test run. I used some crazy math to calculate the diagonals, but you could also throw some butcher paper or a sheet over the window, trace it, and then use that to cut the foam. Hindsight is 20/20.

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 got three yards of fabric, which was barely enough. I recommend buying at least a yard more than you think you need. Lay the fabric over the cushion and make sure its straight. Tuck the corners in neatly (hospital corners!) and put a couple staples at each corner. Then staple the middle of each side, keeping the fabric tight (really tight--I didn't do this well enough!). Double check to make sure it's looking good, then staple around the rest of the cushion. If you want to be fancy you can fold the fabric under at the edge and then staple.

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.