my shirt was a short sleeve t-shirt that was too big for me. so if your shirt isn't too big for you, skip step 1 please.
1. turn the shirt inside out. sew up along the sides to get a smaller shirt. then cut off the extra.
2. cut off the neck of the shirt. for my shirt, i just cut around the regular neck line and then put it on. if it isn't big enough of a cut for you, cut more off. be careful, cus you can always cut more, but you can't put it back on (w/o some extra work). depending on how you cut will also depend on how much it'll be off your shoulder.
3. for mine, i sewed the edges because my shirt was really stretchy and didn't "roll" well. so i overcasted it, which makes a nice look, rather than just a regular hem.
4. i also darted my shirt near the armpits on the front and the back because it was saggy there. turn it inside out and pin it and then try it on before you sew it to make sure it'll fit.
hope that works for everyone.. sorry about the crappy pictures, i never said i was an artist. lol. if anyone does it, send me a picture for sure, i'd love to see it!