The markings on the previous page are important. If the three points are aligned on the wall, 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 thumbtacks.
A quick note regarding placing ink and felt tip pen markings on the underside of your vinyl.
DON’T DO IT!
Sometimes, it may seem unavoidable, especially if you only have access to the underside of the sheet while unfolding it.
Use the masking tape to write or mark on any vinyl surface. Sometimes, you can get away with a small pencil mark. NEVER with a FELT TIP

