A mitre box isn't really the best way to do it though, at least for internal mitres (i.e. those in the corners of rooms). The best way is to scribe one side of the skirting to the other one. That way, if the boards aren't square to each other it doesn't matter as much. You can also arrange the corners so that the scribed pieces are at right angles to the window(s), so they catch the light less, and the gaps are easier to fill, too. And if it's a Victorian-style skirting, most of it is a straight square cut - only the moulding needs to be scribed - and setting it out is easier too.
We've had old properties since we were married, 35 years ago, and at first I tried to cut tall mitres (11" and 10"), with a home-made mitre box (couldn't work out why nobody sold tall ones!), and wasted a fair bit of wood, complete with expensive mouldings. Then a friend took pity on me and taught me how to scribe them, with a fret/coping saw.
Once you know, it's a lot easier than you might think, pretty quick, and the results look neat even when they are cut a bit roughly. Yes, you do need to mitre the returning angles of a chimney breast, but those corners can be pinned so the joints don't gap over time, and one side is really short so it's easy to do.
No I'm not going to go through all the process. It's been discussed at length on the forum, and there are quite a few YouTube videos as well. Search for "scribing skirtings" or "scribing baseboard" (USA). You'll find what you need.
HTH, E.