The stitches that you set aside are half of the sleeve circumference, to create the other half, and to continue knitting the body, you cast on 30 sts using the provisional cast on, and work these sts with the rest of the body. Once your body is complete, you remove the provisional cast on, and work on both the 30 stitches that were set aside, and the 30 sts from your provisional cast on for a total of 60 sts for your sleeve.
I would recommend using a crochet hook in the same size, or one size larger than your knitting needle, and crocheting a chain that is 36 chains long. Turn the chain over, and pick up 30 sts from the small bumps in the back of the chain using your working yarn from the body of your sweater. Once you have picked up the 30 sts, continue working around the body with your working yarn. Repeat for the second armhole. When you are ready to release your provisional cast on, carefully unpick the chain, placing the stitches onto your needle.
You can see a photo tutorial for this technique at the following ehow link:
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All my best,