Place a level width-wise on top of the toilet bowl. Push shim pieces under the bowl where any gaps exist between the base of the bowl and the bottom of the toilet.
Continue inserting shims and checking the level on the toilet bowl. If, after you have filled all of the gaps between the toilet’s base and the floor, the toilet still does not sit level, loosen the nuts on the closet bolts with an adjustable wrench.
Insert shims on the side of the bowl that sits lower than the other, stopping once the bowl sits even. Tighten the nuts on the closet bolts, alternating each side you tighten until both nuts are tightened completely and the bowl sits level.
Trim the shims off at the edge of the toilet bowl’s base with a utility knife. If the utility knife cannot cut through the shims, cut them off by pushing a sharp wood chisel through them.