Advice for hanging a door into an existing out of square jamb.

I have to hang a door tomorrow where there is an existing door jamb in place, but no door stops. I have been told that the jamb is out of square. Just looking for some advice, would you 1: Hold the door in place then trace a line around it and plane it down to size accordingly (allowing 3mm either side etc),or 2: Take the measurements of the door jamb, then transfer them to the door and plane/saw the door down to size that way or 3: another method?

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