I will give you a few tips on how I did it--
First, read this tutorial by At Second Street
Then, here's what I did:
Washed the sweater in hot water and dried on high heat to get it to shrink as much as possible. It's much easier to sew and work with if the fibers are tight.
Cut a circle out of the back measuring 6.5 inch diameter
Cut a strip off the bottom of the sweater 22 inches long by 4 inches wide.
Using a 1/4 inch seam allowance, I sewed the strip ends together to make a band and pinned and sewed that around the circle. Mine did pucker a little as the sweater stretched a bit as it went through the sewing machine, but you can't tell too much when it's on my head.
Cut a brim out of the collar of the sweater. I used the bill template from the other tutorial as a shape guide. (mine came out to about 8 inches wide, and 3 inches at the part where it sticks out the farthest. I stitched around the bottom edge so it wouldn't unravel. I didn't use any cardboard or interfacing (though if your sweater is on the thinner side, it's probably a good idea...plus then the edges won't get stretched out in the sewing machine) Then I pinned and sewed that where it looked good on the hat.
I took the buttons off the sweater and sewed them by hand to the sides of the hat.
In case you missed the post last week, here's what it looks like finished.
And there you have it. If you've got a sweater that's doomed anyway, it can't hurt to try! Plus, it will be even cuter because it will be on *your* cute head!