You can press 4 at the time of taking or during the call. This will play for all parties on the phone. Press 4 again at any time to end the recording. A copy of the recording will be saved in your Google Voice inbox. Hanging up will also end the recording.

Summary:
Receive the call on your Google Voice number. Press 4 on your phone handset. Listen to the automated "Call recording on" message. Continue talking as your call is recorded.