How can I shrink a shirt on purpose?

Hot water if it is not polyester and if it hadn’t been pre shrunk.
Just know that fabrics like wool could over shrink and it might not shrink in the proportions you want it to. If it’s something you love I would see a tailor instead.

The shirts are 100% cotton, but somehow the hot water/dryer trick didn’t work. There is no indication on the labels that they were pre-shrunk. Is there something else I could try?

I don’t mind if I end up ruining them. I can’t wear them as they are anyway.

If they didn’t shrink in the dryer they aren’t going to shrink.

Heat is the way to shrink them, so the hottest water and hottest dryer setting are the best way to shrink them. Run them through the hottest options one more time, and if that doesn’t work, you won’t be able to do anything further. Stretchy knits are more prone to shrinking that the weaves that don’t stretch – think T-shirts and sweats vs. button-downs and denims.

If they were expensive and/or you really like them, another option might be to bring them to a tailor and inquire whether they can bring them in a bit. I have not had anything tailored, so I can’t tell you how much it might cost, but I imagine it would be less than a new shirt.

The shirts are probably toast. Sound’s like you’ve already shrunk them as much as possible, and they only stretch back out when you wear them.

I’d suggest trying layering them (wearing something smaller under them during cooler weather) or donating them to charity. If they were very expensive to begin with, you might find a re-sale shop that will take them on consignment.

Well, I think the shirts just won’t shrink any further, if they shrank at all. Thanks, @ibstubro, for your suggestions of alternate uses for them. No, they were not expensive. I think I may just use some of them as loungewear. lol