Turn the shirt inside out
Put the shirt on and mark the desired hem length with pins or chalk
Remove the shirt and lay it flat
Measure and mark an even hem allowance
Cut off excess fabric if needed
Fold the raw edge up to the marked line
Press the fold with an iron
Fold again to enclose the raw edge
Pin the hem in place
Sew around the hem with a straight stitch
Remove the pins
Trim loose threads
Press the hem flat again