measure around the top of your arm (where you want the top of the armwarmer to reach) and around your wrist.
determine how long you want your armwarmers to be and add a few inches for hemming. if you don't mind raw edges, you can skip hemming all together.
you're going to cut some trapezoid shaped pieces. the widest end should be half of your upper arm measurement, and the skinnier end should be half of your wrist measurement.
cut through both the back and front of the t-shirt so you wind up with 4 trapezoids.