Start by putting the corner cabinet in position. Use your spirit level, from corner to corner, and set the adjustable feet on the cabinet to make it level. Now place the oven cabinet next to the first. Make sure the two cabinets are level with each other, use the feet on the second cabinet to line them up. Bring in the end cabinet and do the same. If you have drawers on your end cabinet, allow for a 20mm setback so that all the cabinets remain flush.
Double clamp the end and oven cabinets together at the top and bottom. Now use your driver, with a countersink drill bit, to pre-drill pilot holes into the top and bottom of the cabinet. Then drive your 28mm screws into the holes to pull the two cabinets together. Use the same process to attach the oven cabinet to the corner cabinet.
Bring in the next cabinet and place it on the other side of the corner cabinet. Level it using the spirit level and adjustable feet, then clamp it, drill it and fix it. Bring in the final corner unit and the end cabinets. Make sure the three are level and lined up. Clamp them together, drill the countersunk holes and screw them together.