a pair of scissors
Trace your foot with a marker on cardboard.
Cut the shape out and place on the sweater and cut around it.
put your feet from the end to the collar in each of the sleeves of the sweater. Cut, after you decided how long you want the slipper to be.
When done, just knit the foot piece to the end of the sleeve you cut off.
Optional: white yarn can give it a more festive look.