Nothing too fancy honestly. Most of the levels I placed stuff semi-randomly first, then played it once or twice to see what happens with the ball, then went back to level design to make it a bit more interesting, then playtested again, moved stuff around again, rinse and repeat until the level felt like fun, not too hard and not too easy, and doesn't look like shit. I also tried to incorporate some unique mechanics every 2 levels or so, so that it progressed incrementally.
In the end I'm proud how it turned out :)