To shrink them I either let my wife wash them
but seriously if I want to take a adult small and make it smaller for a youth I wash it in incredibly hot water and dry it for 30-40 minutes.
I did a bunch of these since I ran out of some youth tees. This dropped the size about 2 sizes. Shrunk the 100% cotton ones pretty good.
They looked like a 12-14 youth size.