Remove the old fabric and use as a pattern to make new. Hope it turns out great
Yes remove the fabric, use as a pattern. If these are old stool you need to have a fire retardant fabric. You can buy a Calico fabric to put on first then cover with your chosen fabric. This can be stapled to the seat base. If your seat base can be unscrewed then do this first as it will be easier to cover unattached. Lay your fabric rightsides down, then foam pad then seat. Staple in the middle on one side, then again on each side. Then again each side of the first staple and finally mitre the corners, stretching the material as you go. To mitre you pull the material over the corner and staple, then you bring the remaining fabric together into the middle of this and staple.
Great idea from Kelli.