Thank you for your answers. After much swearing, I managed to get it right. It's the same principle as with straight corners, except that it just happens that the angle becomes half of the right angle (90 - 45 = 45). Although I succeeded, it's still a bit annoying that there are small gaps between the moldings. I foolishly bought pre-painted white ones since the ceiling was already repainted. Can I fill them with acrylic caulk without painting afterward, or will it look awful? Is it worth waiting a while, or can one assume that the moldings won't move much (they've been at room temp. for 2 weeks before installation)?
Fill in with sealant, it works fine. Sure, there can be a slight difference in shade but it's nothing noticeable in my opinion.
If we're only talking about 90° corners (inside and outside), it's not a bad idea to take two scraps and cut them in all four positions (from the left, from the right: 45° to the left, 45° to the right with a saw) and clearly mark them with up and down (as well as whether they were inside or outside corners: meeting left, right). That worked for me until my father came and tapped my fingers and turned the lists around (so they lay naturally in the saw). Then everything became the opposite. From left became right and 45° left became right and vice versa. But as I said, templates are not entirely bad. Another memory rule I used was that two pieces form a V in inner corners and an A in outer corners.
Well, good luck and hope you don't get more crooked corners like in H_L's case
Well, good luck and hope you don't get more crooked corners like in H_L's case