compound mitre-saw (powered) - you can pick these up from the DIY sheds really cheaply, or Screwfix (for £25 - yes £25!!) or from Axminster (they do one for £70) or a big DeWalt for £650 and all price levels between.
manual mitre-saw - about £15
mitre-box - £5 to £10
marked-out then cut 'free-hand' - only for the experienced
Axminster do a Coving & Cornice Mitre Jig that'll fit most hand or powered mitre saws (£11) - it's purpose is to keep the cornice in the correct position whist cutting.
So, you've never done it before? Then it's the 1st 2 options + the jig; practice on scrap timber 'till you are confident. Tip - mark the real cornice with pencil arrows to remind you of cut directions as it's easy to get them wrong.
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