So, in order for your level to get approved for use in SLAM, you'll have to get a couple good reviews.
Anybody is allowed to review any level, although there are standards which you should follow.
1. While reviewing, you should point out what is wrong and why and suggest ways to fix the thing in question.
2. You should tell the creator if the level breaks any of the guidelines for the collab.
3. Don't be mean to the level creator while writing your review, nobody wants to listen to somebody throwing insults at them.
4. Preferably refrain from inserting your own preferences into the review, try to stay objective. Remember that different people enjoy different kinds of levels.
5. Have a look at this: viewtopic.php?f=89&t=16418. It's got a bunch of good tips!