Well, I actually kinda have a working way for SP level-making:

1. Make your level.
2. Play your level.
3. Play some good levels (usually devres or UD in my case).
4. Play your level.
5. Rate your level from 1 to 5 (only integers, so possible ratings are: 1, 2, 3, 4 and 5).
..a) if rated 1 - delete the level, repeat actions from point 1
..b) if rated 2 - delete the level, repeat actions from point 1
..c) if rated 3 - delete the level, repeat actions from point 1 (I was trying to save those few times when I saw ways to improve them, but it doesn't work)
..d) if rated 4:
....- if you can see room for improval, improve it and repeat actions from point 2
....- if you can't, delete the level, repeat actions from point 1
..e) if rated 5 - keep it, go to point 6
6. Now you have a level. Only 29 more and you're done with your episode, hooray!

Well, at least it looks like that in my case. Results in a lot of deleted levels, yeah. Sometimes it takes time and some online tests (for critics ofc, not for actual testing) to understand it's below 5. For me it's going to take much more than half a year as Jake suggests, because atm I don't have even a single level for my episode.