My parents used to print t-shirts. This led to a stash of white t-shirts in large sizes that none of them knew what to do with years later. After saving them from the dump, I stenciled a shirt for my dad. There are tons of ways to do this, but my (rather messy) way involves:
Black fabric paint
A small paintbrush
A graphic shirt to measure
A shirt to paint
And some cardboard or a file folder.
Lay your shirts flat, and measure the distance between the design and the collar. Also measure the distance between the armpits and the center line of the shirt. Use this measurements as reference for centering your design on the new shirt.
Stick some cardboard (or half a file folder) into your t-shirt underneath where the design will be. This will ensure your design doesn’t bleed through to the back of the shirt.
Trace your letters, one at a time, onto your shirt using the sharpie and stencil. Use the ruler to ensure your letters are level and spaced evenly.
Use the remaining cardboard or file folder as both a palette and a straight edge. Paint along your stencil tracing, covering the parts your don’t want to paint.
Continue patiently painting until your design is done. Let dry.