Sewing/applique question

aylafsu88

I just bought a cute iron on applique from etsy.  The seller recommends sewing it onto the shirt which I think I can do. Does anyone know what you put on the inside of the shirt (i.e. to prevent the thread from coming lose or scratching LOs skin)?  I cannot seem to figure it out by randomly googling phrases online and this is my first attempt at a sewing craft.  Thanks!