Countertops are measured twice for a reason. The first measurement gives you a price; the second, the template, is what the stone is actually cut from. We template after your cabinets are installed and level, because walls are rarely straight and corners are rarely square, and the stone has to match the room as it really is.
Before templating day, have your sink, faucet, and cooktop picked out, ideally on site. Cutouts are made to the exact fixture, so a model change after templating can mean recutting stone. It also helps to decide on the edge profile and backsplash height in advance.
During templating we mark where seams will go, how far the top overhangs the cabinets, and where each cutout sits. This is the best moment to ask questions. If you care where a seam falls, or how the veining runs across an island, say so now while it's still lines on a template.
On install day, clear the countertops and the path from the door to the kitchen. The crew sets the pieces, levels them, bonds and color-matches the seams, and mounts an undermount sink. Natural stone is sealed before we leave. Plumbing hookups usually follow once the adhesive has cured, and your estimate will say who handles them.