I have sewn new covers for two Ikea Marius stools. It's look improves so drastically with a little color! The stool is very cheap and I really like the legs.
How to do it: Cut out a round piece of fabric -or two halfs of different fabric if you want - which is approximately 5 cm wider than the seat all the way around. Zigzag an elastic strap on all the way along the edge - stretching the elastic out as much as possible. Place a piece of batting (I just learned that word) on the seat which is cut the same size as the seat and slip the cover on. Done!