Mask off the top and bottom edges of each stripe, the paint will go where the shirt is exposed and not where the tape is placed. I used a ruler to evenly space the tape and make each stripe the same thickness.
Tape together several of the grocery bags, large enough to fit into the t-shirt and pull it slightly taught. This will make it easier to paint and prevent any leaking though to the backside of the t-shirt.
Paint each stripe in the color and pattern of your choice!
Let the paint dry for at least an hour and then remove the tape slowly.
For fun variations, try different thickness stripes and spacing.