Don’t hang them individually. Instead fasten a run of two or three cabinets together on the floor or another level workspace. This allows you to adjust the mating surfaces for a near seamless fit without having to fight gravity. Align the frames, shim if necessary, clamp into position, then fasten the cabinets together with screws.
Do use a ledger strip. Determine the correct height for the bottom edge of the cabinets then draw a line using a bubble level. Mark the wall stud locations, then nail a 1 x 3 into the studs aligning the top edge of the wood strip with the level line. Leave the nails a bit proud (protruding) to make it easier to remove the strip after installation. Set the cabinet bottoms on the ledger strip and push them into contact with the wall.
Secure the cabinet assemblies to wall studs with screws or nails making sure they pass through the cabinet mounting strip. Watch the video for more details.