It was pretty easy to do! Take a t-shirt that fits you well, and lay it on top of the big one as a template. Cut around the body of the shirt (approximating where the armholes go) giving about half an inch extra on each side for seams, and an extra inch at the bottom hem. Take the newly cut off huge sleeves and cut the seams to open them flat. You'll want to make the sleeve about the same size/ length as the sleeves on your template tee, so do a measurement (plus half inch) to see how much you'll cut off the top (at the shoulder seam). Lay the opened up giant tee sort of on top of the opened up sleeve, the shoulder seam of the shirt where the shoulder seam of the sleeve will be. Trace the shape of the armhole onto the sleeve, cutting the rounded top to match the shirt armhole's shape, and cutting off whatever is required from each side of the sleeve. Sew the sleeve to each armhole, right sides together. Sew the sides together, right sides together. Trim extra fabric at the seams. While still inside-out, sew the hem up about one inch. Done! Easy! Sure, it would look a bit more professional on the inside if I had a serger, but so what? Who's going to be looking at my inside seams?