they have a very thin layer of plastic on the back side you place them onto another fabric like a stocking or a t-shirt you heat up your iron and you "iron them on" the heat melts the plastic and fuses them together :)
If u do win this plz sew it on its better way and last longer then ironing it on. I had a big eagle on my jacket and I iron it on 2days later it was coming off the jacket. Sewing it is not hard to do. I been sewing patches on stuff for 3+ years