10 tips for installing a composite door on your ownStep 1. Be careful not to measure the door too tightly; otherwise, the internal gaps will not be accurateStep 2. Ensure that the cill/thresher is straight, without any bends or dips Step 3. Make sure the opening is square by placing a level on the top, bottom, and sides of the brickwork Step 4. If the top, bottom, and sides of the door all touch the left side of the bubble on the level, it would be preferable if the door was installed with the l