I've done several by pinning waste canvas on the front of the shirt. That gives you the squares you need. It is removed when the stitching is finished. Waste canvas is the name of the product, it is not just a scrap of fabric. Hope this helps.

I've done the same as marshair with waste canvas, though usually I've done hemlines of skirts and collars of blouses, not tee shirts or sweatshirts. I've also done tote bags, which works on the same principle.

I do suggest that if it's an item that will get heavy wear or be washed often, I find that using fusible interfacing on the back after stitching helps keep everything "locked" in place as it were.

I'd also suggest not choosing super intricate patterns or cover-alls for a piece of clothing, as you may want something that will finish a bit faster. It would be rough to work for a year or more on something that was rarely worn and seen thereaafter!

Good luck with your project!

Simple designs with a minimum number of colors and a design of about 4-6 inches, or so. Certainly you could adapt patterns...I designed several and once adapted a favorite bear... 😊

Oh, I just remembered something that might help, too.

I have a friend who transferred a grid pattern onto a tee shirt (temporary, I think she used pencil and a ruler but I'm not entirely sure) then filled in a cross stitch design using dots of puffy fabric paint. Much quicker than stitching, she got the design she was looking for, for her toddler to wear to school the very next day. Might be an option depending on what you need to do.