His old slippers were made out of scrap fabric I had with cats on them.
The new ones, which are considerably larger (they have an inch of space in the toes for him to grow into) have little Doctor Who things on them, like a Tardis, Dalek, etc.
I also had some space fabric so I used that for the soles. He likes them, but they're really for when he grows a bit more. Since he wanted to wear them now (because why wouldn't he?) I cinched the elastic much tighter so they'd stay on his little ankles.