This tutortial shows how to make ruffles & add them to a t-shirt. They used men’s white v-neck undershirts (new of course), but those are pretty thin. I’d use a regular t-shirt, but it’s up to you.
Click title for link at AllFreeSewing.com. You can use the ruffle technique to make ruffles to attach to all sorts of things. It’s a good basic technique to learn & it’s not hard at all.