For any high school student in an activity, letterman letters are an important part of showing off accomplishments. Letters are given away and sold to be sewn to a letterman jacket, so letting them sit around does your athlete/club member no good. Sewing them on is easy, and can be done in less than an hour.
If you have a sewing machine, you're one step ahead of the game, but they can be done by hand as well. The first and foremost step is to set everything out. Find the spot you want to display the letter and make sure it can be seen when the jacket is being worn. Typically, letters themselves go on the left breast and numbers go on the sleeve just below the shoulder, while names, full words, and anything left over goes on the back. For more accuracy, you can set the letter in place and either safety pin it down or trace it with a white fabric pencil (the markings come off with a cloth and water).
Next, you need to purchase some sturdy thread. Your best bet is to find thread that matches the school or the jacket colors. If you can't get a hold of those colors, then black or white thread will work. The needle you use is important as well. Make sure it's thick and sturdy enough to make it through not only the letter, but the thick jacket material as well.
If you're using a sewing machine, thread the machine as you would with any other material. Make sure your line of thread is being sewn close to the edge of the letter so you'll keep a straight line. The type of stitch is up to you, but a whip stitch is recommended. Don't turn the entire jacket around as you stitch; just rotate the fabric around the letter.
If you're hand-stitching the letters, a whip stitch is still your best bet. Be aware that there are numerous layers of the jacket for you to get through as well as the thick letter itself, so get comfortable while you stitch. Start from a corner and work your way around the letter, forming tight stitches to ensure that the letter will stay on the jacket.
When you're done, make sure you knot the string tightly so the stitches don't unravel, and look over what you've done. If it seems that a stitch is loose or missing, go over that area with an individual stitch, just to be safe.
After the letters are on, don't wash the jacket anywhere other than a dry cleaner. Aside from that, just keep earning the letters and repeating the process.