Start out with a t-shirt that is too big for you.
Fold the t-shirt inside out and lay it down on a flat surface. Place another t-shirt (one that fits you properly) on top of the large t-shirt.
Trace the outline of the smaller t-shirt on to the larger t-shirt with chalk.
Place dressmaking pins along the outline, making sure to catch both sides of the t-shirt.
Pin right along the outline so that you have the outline of the smaller t-shirt pinned into the larger one.
Cut along the outside of the outline leaving a 1/2" margin.
Using a sewing machine sew along the outline removing the pins as you go.
Turn the t-shirt back inside out and you're all done.