To laminate properly the surfaces must be very flat to avoid gaps. For large stones, we used power equipment, in particular, a Blackcat which is a diamond grinding wheel on an angle grinder. It still required laying a steel flat edge over the surface repeatedly to find the high spots, mark them, and grind again. Eventually, the surfaces matched and we were able to drill holes for stainless pins and epoxy them together. The pins are necessary as stones can separate, not on the epoxy line but on the next layer of crystals.
On a smaller scale, you can use a concrete block with some sand on it and rub the stone on it. This goes fast with softer stones but does take patience with marble. All the high spots are ground down for a very flat surface before gluing.
Lastly, if the carving is substantially done you might consider slightly “hollowing out” the middle of the stone. Then you will only have to flatten the outside “rim” of the stone. For a base to be flat I might remove just ¼” of the center of the bottom.