The supplies: a sewing machine (borrowed with necessary accessories), sweaters (Goodwill), scissors (a good pair), and a lumbar pillow (to slip the new cover over).
Step 1: Once you have found sweaters in the colors that fit your space and your liking, pre-wash them and then flip them inside out.
Step 2: Trim the sweater at the arms, leaving an inch or so above the armpit point, to trim later.
Step 3: Create a seam across the body of the sweater, from armpit to armpit. You can do this in a variety of ways. I chose to keep it simple; in my house these pillows won't get a lot of use as pillows-P moves them, and I use the larger pillows for comfort most of the time. If your pillows are going to get more use, you want to make sure you really create a strong seam-If you have a serger, use that to create the seam, it will wrap the edges so you won't have to worry about them unraveling.
There are probably several ways to make your seam. Do what you feel is best for you.
Step 4: Flip the pillow right side in, and put your lumbar pillow inside of it.
I chose not to seam the bottom. That way, in the winter when you are putting the sweaters on your pillows to warm up your space, it is easy. No fuss, and you don't have to deal with the hassle of sewing on zippers, etc.
Oh, and by the way, this is half of the prize for the winterize giveaway! The other half is a candle in a luminous holder to add light, ambience, and warmth to your winter space. If you want your space to feel super cozy, you can enter to win here, and if you don't win now you know how to make your own sweater pillows!
Good luck, and happy DIY-ing!