Translating the website via Drupal's interface is fairly intuitive. The following are some screenshots to guide translators through the interface.
Assuming one has translation privileges, at the top of the page below the page title is a series of tabs. Do NOT click on the Edit tab EVER. Click on the Translate tab to be taken to the translation page. If there is not a Translate tab, you either do not have translation privileges or the page was marked as non-translatable by the administrators.
In the situation where there is not any translations of a page yet, the add translation link will appear. If there is already a translation for a specific language, the edit link will be present.
Click on the add translation link to add a new page. This will bring the translator to an editor that has a copy of the current text but other settings predefined to the selected language.
Note that any translator who does not follow these instructions and ends up breaking the main English source will see their translation privileges revoked until they pass a test to demonstrate proper translation knowledge.
Menu translations work much the same way. You go to the primary English menu item and choose the translate button. Note that you NEVER add a new menu link when doing translations.
From there, you can either edit an existing translation or add a translation if there is not one.