Cutting cornice/installing problems

I am having problems getting the bottom edge's of one piece of cornice lining up with its mate on internal corners.
I have a proper mitre box and cutting exact.
I read somewhere to start with the shortedt pieces 1st. Does this mean I put all the short sections up 1st then fill in with the longer pieces.This is what I have been doing on small rectanguler rooms like the toilets and built in robes for practice but
when it come's time to roll in the long bits I would have to bow them or cut off 5-7mm to get them to roll in which creates a bigger than usual gap on the bottom wall edge.
Usually I would pick any wall to start then work around the room.(have since learnt there is a correct procedure)

I have also had problems with the cutting in that if I had to trim off 10mm I would lose the 10mm off the bottom edge but the middle would only lose 2mm but I suspect that all this trimming I have to do after I have cut the cornice to exact length +/- 2mm is related to my install process I have been following.

Help!


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