Simple. Just register an account, select some languages in your profile and start translating. Glossary will offer tips when necessary. You can also add terms to it.
If you're a native speaker, it's best to start with "suggestions". If the suggestion is good, accept it. Otherwise, edit it properly and save the string.
If you're not a native speaker, it's probably easier to start with strings marked for "review". If you're sure of your translation, save it. Otherwise, leave it as "suggestion" for native speakers to check on later.
Automatic translation note:
"Automatic translation" feature is provided as a (possibly) helpful reference for translators, not as a replacement for them. Don't copy/paste mindlessly. If you can't translate some string yourself, skip it, leave it to someone who can. Do not produce bad translations, it is not the purpose of BGforge.