Cut the skirting in around my new fireplace, scribing, fixing etc
I would begin with a cardboard pattern. You could measure each distance, lay them out on the right edge of the cardboard. Or you could use  profile gauge such as this one.



I would then do a rough cut of the cardboard, lay it into the corner, and trim to precision. Do not cut the left edge until the end. Then transfer the profile onto the molding.

On the actual molding, I would also leave the overall length a bit longer than the pattern, maybe by 1/8 inch. This gives you room for tweaking and the mitre cut is easier to shorten than the profile.

A jigsaw is fine to cut the profile. You should back cut the curve, that is, trim off a bit more on the backside than the front. This makes fine tuning easier and avoids the chance of seeing the cut face in the finished piece.

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