Does everyone know what DIY stands for? I had someone ask me that once. Do-It-Yourself.
Cut 4 equally sized strips of your fleece/flannel/felt. (I recommend creating a simple pattern for yourself using wax or parchment paper. Or wrapping paper. Or anything.)
Put on your shirt and decide where you want your top strip and bottom strip to sit. Loosly pin those two on and carefully remove the shirt (and put on another one). Lay the shirt flat and now adjust your top and bottom strips so they are level, and pin in place. Evenly space the middle 2 strips and pin in place.