The button goes on after you felt them. You do a small swatch of your yarn separately from the slippers, mine was about 4" x 6" and you felt that too and use it for embellishment. I did a small disc of felt under the button just for costrast.
The pattern is screwey so you will find yourself winging it, I did. I'm glad I didn't use the Monos yarn they called for. I used Patons and went a size up because the gauge is way off. There are hundreds of people on Ravelry who made them, it was really helpful to read their comments and see what yarn felted well and what didn't. I could have made these easily in a day if I didn't have kids hanging around! They are super fast!