I needed to replace a vanity in a bathroom with 3/8″ tile over 1/4″ Hardibacker board.  That meant dropping the vanity into a deep hole.  How do you shim something like this?  It was especially important to put quite a few shims along the sides and back of the counter because the countertop was to be a 3″ think marble slab.

What I did was to mark the top of the counter on the wall, then level the counter and mark the walls again – one side and the back.  (Just held it up level, nothing fancy.)  Taking the difference between the two marks I was able to cut shims for the four corners.  (There was a substantial difference from left to right, minimal front to back.)  Dry fit showed perfect leveling. 

Then comes the good part.  I wanted plenty of shims so the heavy top didn’t concentrate too much weight on just the corners.  So I cut up a thin plastic merchandise bag (mine was orange…) Then I mixed up some epoxy putty and had an assistant take blobs of it and wrap it loosely in the plastic pieces.  You don’t want it too tight or the epoxy might not fully compress.  And you want to use large enough pieces of plastic so the epoxy won’t mush out and permanently stick your cabinet to the floor.  I guestimated how much I’d need in each little packet based on the thickness of the nearest corner shim.  I put the little packets around the perimeter of the hole and set the countertop on the shims.  The bottom of the counter pushed the epoxy blobs down, making them self-adjusting shims.  When cured (4 hours for this epoxy) my counter was level and very sturdy.