Steps to start translating:

1. Register at http://l10n.privnet.biz
2. Join the group for your Language (if there is no group ask rak here to create it)
3. Now on the translation page you can export an tar.gz with all the po files for your language you can use the tool of your joice to translate them, I like poedit under windows others use kbabel under Linux or just a Text editor.
4. Import the po file by uploading it at the import screen. If you don't check the mark as sugestion box new Strings from you will get into the translation. Strings you changed will be marked as suggestion an will require someone to aprove them (depending on the Settings on your language you can do that yourself on the edit screen)

as an alternative to 3 and 4 you even can start translating on the edit screen which is pretty easy but not that fast.

5. Try to communicate with your other group members to prevent double work

That should be it.

Spuky