If you plan to coat the floor someday, tell me before we pour, because moisture rising up through a slab is what makes coatings peel, warp or discolor, and the time to deal with it is with a vapor retarder, a sheet laid under the concrete. From there the job runs in order. We break out and haul off the old floor, dig down to firm ground and pack the base so the slab has even support everywhere, since a base that's only raked flat settles and takes the slab with it. Then we set forms so the floor falls slightly toward the door, place rebar, the steel that helps hold the slab together, pour and finish, cut control joints, shallow grooves that give the concrete a planned place to crack, and keep the slab damp or covered while it cures. If the budget gets tight, the base is the last place I'll cut, because nobody sees it on pour day and it's what decides whether the floor stays flat.
Some people want more than plain gray, and stain or a pattern is decorative concrete work. Most slab estimates around Los Angeles leave staining and stamping out, so price a decorative finish on its own. If the garage sits on a slope or the ground beside it needs holding back, we also build concrete retaining walls.