I used two t-shirts (one adult & one toddler). The slipper pattern was from Burda. I didn't do a second layer for the bottom/sole, and I added a 5" strip of elastic for each slipper to help hold the slipper on. If I were to make these again, I'd make the slipper come further up onto the top of the foot. Overall, these worked up super quick and easy.
As an FYI, my DD modeled these and she wears a women's 9 1/2 - 10 shoe.