First off, you’ll need a Launchpad account with your languages set up.

Go to http://launchpad.net, and click "Register" in the top right corner. Follow the steps to create an account. Once you’ve created an account, go to your page by clicking your name in the top right corner. Click the yellow "Edit" icon next to the "Languages" header, and add all the languages you intend to translate into.

Once you’ve done that, you’re all set up and ready to go. Go to http://translations.launchpad.net/ubuntu-manual you should see the languages you’ve selected listed. Click on the one you want to translate into, and translate the text you see into the boxes.

It’s that easy!