Is this actually for cutting skirting boards?
If so, you only need mitres on "returning" corners, such as the outer corners of chimney breasts where they turn back towards the walls. On internal corners (the four corners of a room, for example), you'll get much more satisfying results by cutting one board square and "coping" (also called scribing) the other one to match it. That's cutting the opposite of the profile of the board into its edge so the coped board fits onto the other board's moulding exactly.
Fitted together, it looks like a mitre, but it won't gap as nastily when the boards expand and contract, as they inevitably will. The square cut board goes on the wall furthest from the window and parallel to it, and the coped boards meet it. Work from there back round the room. At each joint, cope cut the board the light runs along, and square cut the board the light shines onto. That way you see any gaps the least, because of the way the light hits them. Coped joints are also forgiving of non-square corners (or can be "adjusted" easily) - mitres aren't.
There are several tricks to doing the coping cut, but there are a raft of how-tos on t'internet, and you don't need power tools. A coping saw is really inexpensive, too.
You do need to mitre returning corners - there's no other way that I know of - but they're much easier to fit than internal ones. Use a bevel gauge to get the angle the two walls meet at - more likely not to be ninety than to be correct. Halve the angle to get the mitre (do this with compasses on a piece of paper if you can't measure it accurately enough). If you're confident, go for it, otherwise make the wooden mitre slightly more 'pointy' (acute) than the actual wall needs - only by a degree or two at the most. It gives you a thin outside edge you can force together, and will gap at the back, where you can fill it.
On returning corners traditional skirtings are held by pins, driven through the face to the back end grain on the other board, and at a slight vertical angle. Doing it from both directions locks the corner together, and the pins are hidden beneath the paint and too far from the thin outer edge to split the wood.
Obviously leave the boards long so you can trim them to length. If you cope the other board (the straight alcove wall), you have leeway to slide the chimney breast board up snug to get the visible mitred corner fitting really tightly. You can't win with the ones on the front - they have to be trimmed right to the fireplace (or across the front if the fireplace has gone), but you have a game chance of getting a good set of corners that way.
Those tall mitre cutting arrangements probably do work, but your best bet while using them is to mark the whole cut accurately and check you're cutting where you expect to be! It's worth investing in a good square (that you've checked *is* square), and a bevel gauge.
Hope that helps,
E.
