I used THIS tutorial, which is awesome. Erin includes templates you can use, and gives great detailed instructions with photos.
I made some more the other day for a friend:
Here are some up-close photos:
For the outside fabric I used cotton, and for the inside fabric (the side that rests along the dog's body) I used a soft flannel.
I found a package of 4 "D" rings at Wal-mart in the button section and I think it was only .78 cents. They can also be found at Jo-Ann or Hobby Lobby.