cutting architrave mitres

One job I have never got on with is cutting mitres. Can anybody tell me if there are any trade secrets to doing this? I've got 4 new door openings to fit ogee type archtrave to, one of the door linings is slightly out of square which is a pain as it means that the cutting angles can't be 45 so need careful measuring. When doing this before I have ended up using a sanding disc on an angle grinder to trim the cuts.
Is there a correct sequence of operations? By that I mean do you cut the top architrave first then the two side ones or what?
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