Vinyl Christmas presents

I have a Cricut Explore 1. My husband wants to get custom sweatshirts for his family for Christmas. I told him its probably too late for them to come by Christmas and would be too expensive to order all of them, but I could make them on my Cricut. Hes worried that after a few washes the vinyl will come off. I’ve never used iron on vinyl. Whats the best way to ensure the vinyl will stay on/any other iron on tips?