With yarn you need to sew it in. I use a 3-5 inch needle. You'll also need pliers to help pull the needle through. With yarn you only have to do every other hole around the head and even less when filling in the rest of the head. It doesn't take long, but it can be a pain on your hand. It's super easy.
Thank you! I use a 3-4 inch needle with a wide enough eye (hole) for the size of the yarn. I also need pliers to help push the needle through. You have to sew through the end and have the needle come out the neck. Once the end is out, tie a knot. It's pretty simple and doesn't take a long time.