I gathered up a drop cloth that I already had on hand and found a pan that was the same size as the stool top.
Traced and cut out the circular top portion.
Measure from the top of the stool to the floor and cut out a strip to be the skirt.
You will want it to be twice as long as needed if you plan on having it gathered.
Set your tension to +3, and your stitch length to 4.5 to make the gather on the skirt.
Stitch the skirt so that it gathers.
Pin around the circle and test the fit before you do the final stitching.
As long as the fit is how you like it serge the final seam.
Then I chose to use a stencil I already had and some fabric spray paint to Pottery Barn it up.
I used some spray adhesive on the back side of the stencil to hold it in place and put a garbage bag over the rest of the slipcover to protect it from paint.