Go to “Challenges” Section
Click the “Open” button located in the top central portion of the web page.
Place your cursor over the announcement photo of the challenge that you wish to join. A “Join” button will appear. Click this button.
Read the Guidelines. Make sure to click the “+” signs to expand the guidelines. Click on the “I read the instructions” box at the bottom, and then click “Continue”.
You may enter up to “4” images per challenge. You can see how many submissions you have left in the lower-left corner of the “Submit photos” box. Choose where you want to upload your photos from
If you chose the “My Computer” option make sure that your submission is at least 700 X 700 pixels.
That's it! you're good to go!
Currently there is no limit to the number of challenges you can join (as long as you meet the requirements such as status).