Over the years I’ve painted a fair amount of baseball shirts in playrooms and bedrooms for children. Living in the Boston area, home of the Red Sox, that is the team shirt I am most often asked to paint.
There are two ways I do these shirts. The first is to lay an actual Red Sox shirt down on a large piece of heavy paper and trace the outline of the shirt. I then use some repositionable spray adhesive to mount the paper to the wall. I cut the shirt outline with an X-Acto knife, right on the wall. The background color is rolled on and then I freehand all the details and shading.
Another option for doing these shirts is to use an overhead projector. Again, I use a large piece of heavy paper sprayed with the repositionable spray adhesive. I project the image onto the paper which is mounted on the wall and trace the entire image. I first cut out (directly on the wall) the outline of the shirt and remove. This is then roller stenciled. Once dry, the shirt cut out is 
replaced and the details on the shirt, such as lettering, buttons, stitches, etc. are individually cut out and stenciled. I highly recommend Projection Stenciling, by Linda Buckingham and Leslie Bird, to learn more about this easy technique.