Hi there Marble Blasters, I'm new to the Marble Blast forums and here's my first custom level: Persistence. I can't make custom interiors, but I can prove awesome levels can be made without that ability.
1) Put the .mis file and the .jpg file into platinum/data/missions or some subfolder (the folder, not the zip file).
2) Put Archstand2.dif into platinum/data/interiors (the folder, not the zip file. If there is no folder, create one). All other interiors used in this level should already be in the game, in interiors_mbp.
NOTE: I'm super bad at adjusting platinum/ultimate times due to my lack of skill at speedrunning Marble Blast, so please write your platinum/ultimate time suggestions in the comments (for now they are set to 8m and 4m respectively, I don't know if they're too easy or too hard or impossible, personally I couldn't do either of these but that's because I passed the level only twice). Also, I know there already is a custom level in the CLA called Persistence, but I got the name idea independently.