To get started taking apart the sweater, you'll want to detach the arms from the body of the sweater. Sometimes it will be clear where to start if you can find the yarn end that was used to sew the sleeve to the body (as was the case with my sweater; because it never fit me I never wove in the ends).
If you can't find a yarn end, simply pull the arm away from the body at the seam. You should be able to see horizontal lines of yarn running up the middle of the seam. Cut one of these threads and you can being to unravel the seam by hand.
Alternatively, use your scissors to cut each of these pieces of yarn as you go around the armhole. This gives you more little bits of yarn to pull out as you unravel the sweater, but it's also faster. Just be careful not to cut the main part of the knitting. If you do, it's not a huge deal, you'll just end up with some shorter lengths of yarn.
Continue in whichever manner you choose around the armhole until the sleeve is separate from the body. Repeat on the other side.