Adhere your trimmed fabric piece to your letter, making sure all edges are lined up as you work. Make sure to adhere glue only to the edges of your letter in a way to create a pocket with your fabric that you will fill with fiberfill to give your project dimension.
Tuck some fiberfill up into the pocket you just created, working with very small amounts of fiberfill at a time to avoid overstuffing and thus causing your fabric to separate from your letter. (If necessary, use a wooden chopstick or other device to poke the fiberfill where it needs to go.) Repeat this step, working in 2-3 sections, until your entire letter is covered and stuffed.