First Thing to do is reading the
Proofreading Guidelines.
Then, click at the P1 link (up right).
You'll be prompted to a page with a list of books. I recommend you choose a BEGINNERS ONLY project. Just click at the title.
A new page will open (called "project page") with the book's info and the project comments. Read the project comments until the end.
Then click Start Proofreading in the end of the page.
And you're ready to go.
After you've proofread the page, click "Save as done" or "Save and go to the next page".
Tomorrow, when you come back to DP, you just need to click "My projects" (top) after logging in and you'll see a link to the book you have previously worked on.