MATCH THE WAVEFORMS
You’ll probably notice when you drag a video clip into your editing software that there’s a corresponding audio track below it. That’s the audio from the camera itself, and gives you a guide as to where to place the track.
Remember that loud sound you made at the start of recording? That will show up as a spike in both the audio from your camera and the audio from the recorder. Match the two spikes, and you’ll be well in sync!