In order to check that your base will be laid square, the diagonals should be measured and be equal (as per diagram). We would also advise you to use a reputable ready-mix concrete company who will also be able to tell you the amount of concrete required.
If you are close to an existing building or boundary wall, make sure that you allow for the overhang of the roof, i.e.., flat roof garages 6 inch (150mm) on front and rear and 3 inch (75mm) both sides, apex roof 6 inch (150mm) front, rear and both sides. This does not include gutter width.
After your garage has been erected we strongly recommend that a sand and cement fillet be placed on the base to panel joint all around the inside (not outside) of the building to avoid dust and debris blowing under the building and it also helps with water ingress.