How do you get a four-legged stool to stop rocking?

Try making a sanding surface you can trust: stick coarse sandpaper to a flat board, then put the stool on it and move it back and forth gently. Check the feet often so you stop as soon as they make even contact. If the feet are angled or wide, go slowly and keep the stool upright rather than tipping it as you sand.

The board doesn’t have to be level with the room. It does need to be flat, since the goal is matching the feet to one plane.

For a larger chair, I’d shim it on the flattest surface you can find until it stops rocking, then mark the feet that are too long. A flat but sloped garage floor can still work. You’re matching the bottoms to that surface, not trying to make the chair level relative to the floor.

Take off a little at a time with a rasp, plane, or careful sanding, and test again between passes. If the chair will live outside, check the fit again after it’s been in its usual spot for a while. The ground and the wood may not stay exactly as they are today.

If you don’t want to keep trimming wood, adjustable feet are the straightforward fix, especially for a chair that will sit on changing outdoor ground. They can be visually noticeable, though, and you’d want hardware suited to the conditions.

If you prefer the clean look of bare wood, try the shim-and-mark method first and remove as little as possible. A four-legged piece can rock because of a small mismatch, so avoid making a big cut based on one quick test.