@wouter52 @alpacalypse re process: I feel like I have a few mechanical "themes" that I keep coming back to ("visit all these goal points in any order," "pull towards you vs push away from you," "two different characters/groups that are opposite relative to each other", "trace or extend along a path"). Also I tend to like wonky mouse controls and irregularly-shaped multi-tile objects. After a while, you get a sense for what mechanics are usually going to be able to create interesting puzzles.
A good sign that a set of mechanics can make interesting puzzles, in my opinion, is being able to build a "one-way door", where it's possible to move a character in one direction past an obstacle, but once they go past they can't come back.
In terms of level design, I usually find the most success by trying to think of one interesting interaction that you can do with the pieces (in this game, that would be something like, "what if we had to push a horizontal bar left and then right to get around an obstacle, using the same vertical bar both times?") and then trying to construct the rest of the level to force that idea to occur. I don't always succeed at forcing the idea to happen, and sometimes I end up permitting a lot of alternate solutions... but if a level is completable and the solution isn't obvious, then that's a good sign that even if there are alternate solutions they're probably also interesting.
Then I have to play through each level a lot, to try to judge difficulty and level order, and to make sure I'm not missing any simple ways to solve it. I don't always succeed at this either, which is why I often accidentally make the last level easier than the ones before, or put a really hard level too early, lol. Recently, I've realized that making a few "real" puzzles first, putting them at the end, and then trying to "fill in" the difficulty curve to teach different interactions in earlier levels, tends to create a better experience. I'm definitely still learning about how to do this better.
Wow, I ended up writing a lot. I hope this sort of answers your question, lol.