For this you will need a chair, a tee with a sweet design, a screwdriver, a pair of scissors, and some attachment contraption (I used a needle and thread).
NOTE: The image above is from one of my better iterations of this DIY. The one documented is from a smaller shirt with a smaller design. You're generally better off with large shirts cause they have larger designs.
Step 1: Remove Seat From Frame
Step 2: Fit Shirt to Seat Cushion
I experimented here with small sized shirts, so sliding the shirt in it's entirety was best, with smaller adjustments to secure it to the seat. However, if you use a large shirt (which ideal as they usually have large designs, more adjustments will be required, including removing the backing of the shirt to remove bulk and obstructions for the screw.
Step 3: Secure the Cover
In my case, I sewed the cover with needle and thread to the point that it was secured to the seat. I sewed the bottom, then the sleeves, and the neck line. Stretching the shirt and cutting unnecessary fabric as I went.
BE AWARE: You want to avoid bunching where the screws secure the seat to the frame.
Step 4: Make Hole for Screws and Re-attach Seat to Frame
This is pretty easy. Just be wary of securing the cover with the frame in mind. You want to stretch the cloth so you don't have anything hanging under the seat or bunching on the sides.
And enjoy :)