How can the seat cushion not slide? I want to make a chair cushion for my son, but I can't stay. I always fall to the ground. How can I not slide down?

This is a short answer. Just sew two ropes on the two corners of the mat and tie them to the chair.

There are cushions like this.

The cushion used on my stool is tied to the leg of the stool.

Well, I hope the above suggestions can help the landlord. Good luck and happiness.