Cut the collar off. This is more necessary if you're going to actually be using this shirt as your reconstruction. If you're only cutting apart the shirt to use as a pattern, you can leave the collar on, if you want.

Mark the front like so. This is the sleeve line.

The top of the line should be about 2 inches down from the shoulder seam. Make a line from that spot on the neckhole to the armpit.

You'll probably want to mark it with pins before you cut.

Now, we cut!

Just cut where you've marked, but ONLY through the front of the shirt. Don't cut all the way through the back. Stop once you reach the pit.

Cut where you marked the back.

Now your sleeves come off!

Cut up the sleeve seam, and this is what you get- a raglan sleeve!

You'll probably want to mark the sleeve, but if you don't and you forget, the longer of the two pointy bits at top goes in back, and the shorter end in front.

Don't throw the rest of the t-shirt way, either. Cut up the sides of the t-shirt, because you're going to need those front and back pieces as patterns, too. You CAN NOT attach a raglan sleeve to a shirt that is supposed to have standard (set-in) sleeves.

To attach the sleeve, sew the sleeve seam up (the one you just cut open), and the sides of the t-shirt. Line up the Longer Point with the back of the shirt, the sleeve seam with the side seam of the t-shirt, and the Short Point with the front, and sew! They're really easy!