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Using a straight edge find the straightest side of the door. Then using either a large framing square or the 3-4-5 method to square the top and bottom of the door in relation to the straight side. It's then relatively straightforward to square the other side from this.
The trick is to do this while taking off the bear minimum from the door, but once you're done you can treat it like a factory door.
Bobbo
This is really helpful, what would you recommend doing if your door isn't a straight rectangle (old door) but the liner is?