Open the Mixer window.

Select a track to record to.

Click on one of the FX slot arrows, and choose Edison from the list.

By default, Edison will record the last 5 minutes of recorded data. To change that, find the Max Recording Time dial towards the top of the Edison window, and key in a desired number of minutes, or bring it all the way down to record as long as you have space on your hard drive.

If you want to apply effects to the recording, add desired effect plugins to the FX slots below Edison slot. That way you will get a raw recording, and allow effects to be heard real-time in the monitors while you're recording.

Hit the Record button in Edison window, when you're ready to begin recording.

Sources:

http://warbeats.com/Tutorials/FL-Studio?topic=13268&name=FL-Studio-Basics-12-Recording#view