If you mean assignment, how you do it is up to you. Static item, book, Word document, Google Docs, Windows Notepad, scribbles on paper, cuneiform in clay... we don't check your work. It's "honor system," but if you're not honest about it, you're only hurting your own process. Sure, you'd get a Merit Badge at the end, but there are easier ways to get Merit Badges.

Once you've done it, you do need to log that you've done it, using the Entry Form, one of the links above (at the bottom, "Important Prep Links."

Now. If by "challenge" you mean the Contest Round, those weekly assignments need to be both logged and, if you want to be considered for the Contest Round competition, you also need to have your work in a static item or book/blog entry and submit it using another form (also in the links above). For more information, check the second pinned post at the top of the discussion forum: "IMPORTANT: How to compete in a Contest Round"  

I hope this helps and doesn't make things even more confusing. Don't hesitate to ask again if something isn't clear.