How to design a sweatshirt?

If you want several of the same design, there are great companies on line that are reasonable. If you are wanting to make your own, I would suggest using a cutting machine such as a Cricut. There are many color options for heat transfer vinyl as well as glitters. There are step by step videos on Youtube that show how to do the process.

Staples has a desighn program you could get to make a iron on then you could sew one iron one paint one what ever you want

I would browse around canva.com at some of their graphic design templates that could give you good ideas.

This is an idea if you sew: Take a sweat shirt , split down middle, sew trim on both sides of the cut edges. Make button holes or no button holes, sew on buttons or attach with Velcro and then use as a cardigan. U can also add trim around the collar. You can do a lot if you sew and do crafts with lace, buttons, trims etc. Can be really cute. You can also cut out shapes and use transfer pellon "paper" to iron on and stitch around and there ya go!