Use two yarns at once, preferably an acrylic because it’s more durable.
Pattern hand written by my Oma translated from German to English:
Use 2 strands of the regular cheap acrylic yarn.
Use size 8 needles.
Cast on 29.
1R: K9,P1,K9,P1,K9
2R: Knit all
3R: Repeat 1R
4R: Repeat 2R
Keep repeating as above (Rows 1-4) until you’ve knitted 3/4 the length of the foot to be fitted.
For toe box: K1,P1 and repeat until you have a piece the length of the foot to be fitted. Stop about 1/2 inch short, as yarn will stretch over foot. Bind off.
You will have a rectangle. With yarn, “sew” up beginning creating the heel. “Sew” up toe box, closing it up to create the slipper. Add pom pom if desired.

NOTE: SINCE I AM NEW TO KNITTING I DIDN’T REALIZE THAT I NEEDED TO CHANGE UP MY GRANDMOTHER’S NOTES SLIGHTLY. THE TOE BOX IS SUPPOSED TO BE A RIBBING, BUT SINCE THE CAST ON IS AN ODD NUMBER THE INSTRUCTIONS SHOULD READ:ROW1: K1,P1 REPEAT UNTIL END OF ROW. ROW2: P1,K1 REPEAT UNTIL END OF ROW. REPEAT THESE 2 ROWS UNTIL DESIRED LENGTH. THIS IS REVISED TOE BOX DIRECTIONS. SINCE I DIDN’T DO THAT FOR THESE SLIPPERS, I DID NOT ACHIEVE MY RIBBED EFFECT.