Sync Audio to the Camera
If you’re separately recording video and audio you need a way to sync the two in post. The best way is to use a production slate. Ben Lucas has a full tutorial on using production slates and another two on how to synchronise audio manually and automatically after you've recorded.
If you don’t have a slate, you can get by with just your hands. Clap them loudly close to the mic and in view of the cameras recording the take. If you’re shooting simple scenes with one or two cameras this will work fine. If, however, you’re filming something more complex a slate will make post-production much easier.