Insert cardboard into the shirt to keep the paint from bleeding through and to stretch it out to allow your painted lines to be straight.
Tape off the stripes on the shirt with the masking tape making the gaps the same size as you go along. The size is up to you. Start from the top and work your way down. Also do long strips of tape on the sides to prevent it from messing.
Paint the open gaps you have sectioned off on the shirt. Do about two to three coats, letting it dry properly in-between each one.