I wish I wrote down what I did when I made slippers for myself (and then for my nephews when they saw me wearing mine "Pleeeeeeeease!!!" "Are MY slippers done yet?" "Are MY Slippers DONE yet?" "Are my slippers done YET?")
Basically, what I did was chain enough to go almost the width of the end of my toes (sorry if that doesn't make sense... The beginning chain went from about the inside of my pinky toe to the middle of my big toe). Then I sc'd across, placing 3 sc in the end and then sc'd across the other side, with 3 sc in the other end. ie: work in the round.
Continue this way, increasing on ends until the round is the width of the foot or a bit bigger.
Continue to work even in rounds (not increasing any more) until the slipper fits up to the top of your foot (where it meets your leg) (not so good at anatomy terminology either...
At this point, you need to start working flat, working from the side around the bottom and up the other side. You'll need to decide how wide you want the opening for the top of the foot to be. Keep working flat until the bottom comes to the back of your heel.
Fold the flat piece so the sides come up together and stitch along the end to close the heel.
I think I probably did a round or two of sc around the whole opening. You can use the round of sc to decrease slightly if the opening gapes.
Wow, I'm really not so good at writing instructions!!! It works, honest, but I don't know if it's understandable!! Sorry.