If the floor isn't level, find the highest point in the floor along the wall where the cabinets will be installed. It’s best to try to start the base cabinet installation as close to this high point as possible -- it's easier to shim under the cabinets on the lower spots to make them level with this first one, rather than starting on a low point and having to trim the bottoms of the cabinets while working "uphill".
If the floor is level, start the install on the corner cabinet, it will help you to stay level through the rest of the installation.
Measure base cabinets and use the pencil and level to mark a level line on the wall from the high point in the floor to the cabinet height. Be sure to use a level to assure this line will be true horizontal. This is the guide to be followed for installation.
Use the stud finder to locate and mark the studs along this line around the room.
For the sink cabinet, use shims where necessary to adjust for level. Measure the wall for the plumbing fixtures (Image 1).
Transfer those measurements to the sink cabinet and using a hole saw, cut holes for the plumbing cutting from the inside of the cabinet (Image 2).
Dry fit your first cabinet into place and check for level across the top. If needed, shim until level.
Predrill through the top framing piece into the wall studs and secure the cabinet into place with screws. Re-check level after securing the cabinet.
Repeat this process until all of the base cabinets are installed. Attach the
doors and hardware after all of your cabinets are in place.