I'm sure these will be modified over time, leave comments if you have suggestions.
The only rule for the designer challenge is to try your best! The designer challenge exists to help challenge and motivate you to step out and try something new. So if you're doing that, then your doing the right thing.
To participate you should:
Since we currently have no way for designers to post their designs on the site, you will need to have an off-site location to host your files if you wish to share them. Please leave an image and a link (if offered) in the appropriate challenge thread. See here for info on posting an image to the forum.