Start by launching the Google Voice Wizard by saying the famous "Google Ok" or by manually launching the application on your smartphone. Then ask it to "Become my interpreter" or "Start the interpreter function" for example.
Once the wizard understands the task you have just submitted to him, he should ask you in which languages you would like him to operate. The first language, the one you speak, is detected automatically.
For the second language of the conversation, you can tell them aloud, "in English", for example, or manually select a language from the appropriate menu.
The rest is very simple, since you and the person you're talking to just start the conversation.