Push the wood shim underneath the bathroom vanity until it holds the vanity level.
Mark the wood shim at the place where protrudes from under the vanity. This is where you will cut to remove the part of of the shim that is visible.
Remove the wood shim from the under the vanity.
Trim the wood shim at the pencil mark using a utility knife, making a shim that will fit flush with the vanity.
Dip a small piece of cheesecloth in wood sealer.
Rub the shim with the piece of cheesecloth to seal it and allow the sealer to cure. The time this takes is referenced on the sealer itself.
Sand the shim lightly using a piece of 150-grit sandpaper.
Clean all the sawdust off of the shim using a clean cloth.
Apply a thin coating of wood stain that matches the wood of your vanity using a piece of cheese cloth, or apply matching paint using a paintbrush.
Allow the wood stain or the paint to dry thoroughly.
Slide the shim underneath the vanity again.