See below for step-by-step directions complete with pictures :)
If the shirt has a graphic on it (like the one pictured below)
Lay the shirt with the graphic facing up.
Next fold in each sleeve to meet the edge of the collar(as shown below )
Next, fold the right side into the middle, and fold the left side on top of the right(as shown below)
Next, sew around the open edges. . . .
After all of the open edges are hemmed,run a zigzag stich through the top where you folded under the sleeves, so that they dont bunch up when you wash them.
(don't mind the brown one, it was my first attempt)