If the three points are aligned, on the wall, on the floor, and outside the door, they will match the sheet vinyl markings and everything will be square.

Bear in mind that the vinyl will want to cover some of the markings. The vinyl when carried in will be rolled up. So only the backing will be visible for mark (B) Visibility will be obscured.

Marking the underside of the vinyl will be difficult. So a simple technique to transfer the markings to the underside of the sheet vinyl will involve a pair of thumb tacks.