Things You'll Need
Take the felt letters or numbers that you would like to sew onto your Letterman's jacket and look at the bottom felt color. The bottom color is the color that you see the least amount of and it goes around the edges of the main color.
Next select a thread that matches the bottom felt color. You want the stitches to blend in with the felt so they cannot be seen.
Using the whip stitch method of sewing, make small evenly spaced stitches along the entire edge of the letter or number. Make sure that you work closely to the edge of the felt and do your best to create even stitches.
If you are worried about the number or letter sliding around and becoming crooked while you sew, take a few pins and secure the felt onto the jacket temporarily.
Remember that high school letters go on the left breast and numbers go on the sleeve below the shoulder. Names and large numbers go on the back, usually curving slightly in a rainbow shape.