At Bitwave I had the role of a level designer working on Gimmick! 2, As a part of that position I worked independently with weekly playthroughs of leaves I worked on. These levels I would usually box out a general shape within the size limits we had on each level using gray boxes to get a feeling for how I wanted the player to move through the level. 

After I got a boxed level I started working with the tools and mechanics we had in the game to create some challenges, secrets, using collectables to guide the player and creating fun and interesting gameplay. 

Once that was done I played through the level testing things out in the editor, reworked things I felt were lacking or uninteresting and then I did a final pass for all the more technical things (checkpoints, camera bounds and so on). 

Usually at this point I playtested the level with my lead, letting him play through the level and give feedback as I took notes of both his feedback and things I found were unintentional in my design to then go back and finish up my work on the level. 

But I also did a lot of testing with our mechanics and how they interacted with each other and other environmental actors, often looking for bugs and unintentional reactions between them and so on.

Some of these mechanics were made by me as well as I did some scripting as a part of my position at Bitwave.