Try on the T-shirt and mark the desired length with pins or tailor’s chalk
Lay the shirt flat on a clean surface
Measure from the bottom hem to the marked line
Cut off the excess fabric, leaving a small allowance if you want to hem it
Fold the raw edge inward twice to hide the cut edge
Pin the fold in place
Sew a straight stitch around the hem
Use a zigzag stitch or fabric glue if you do not want to sew
Press the hem with an iron
Try the shirt on again to check the length