You can choose to buy pre-cut rag quilt batting squares to sandwich between your fabrics. You can also buy the bulk kind and cut it yourself but it's much easier to buy the batting squares for rag quilts already pre-cut. Let someone else do the cutting for you so you can sew up your rag quilt gifts super quick! I like to use batting instead of using another layer of flannel because it reduces the bulk and makes it much easier to sew. I also like the fluff that it gives the quilt. If you use batting between two pieces of flannel, remember that you will be sewing four peices of flannel together as you put your rows together. If you choose to use flannel in between that means you will be sewing six layers of flannel together. That'll also give you much more work when it comes time to snip the edges of your rag quilt. There are many different sizes of pre-cut batting squares. Make sure you leave enough room to sew your squares together. I like to leave a space of about an inch on all sides. It depends on how long you want your ragged edges on your quilt. Experiment and you'll find the batting size that works for you.