Secure the Remaining Cabinets
Pre-drill holes along where the cabinets will be attached together. Sink a screw into those holes.
Put shims in the space between the wall and the sink cabinet, along the stud lines (Image 1). Sink drywall screws into the wall, through the shims, securing the cabinet to the wall (Image 2).
Often there are gaps where the cabinets meet the wall, preserve these gaps because walls are often uneven, so the gaps keep the front of the cabinets flush and will be covered later by the countertop.