How do I enter a challenge?

You just open that challenge thread... And make a
"This is my starting day of this challenge" kind of post.
Then next day you go to that thread and type a "Day 1 completed" kinda post.
Then the next day you do the same and make a "Second day completed" post.
Etc.
Etc.

When you complete all the days of that challenge, you edit your signature to include it.

 

Might I suggest you join the Batman Challenge, @Bonhart