This video tutorial by photographer Nathaniel Dodson (from the website tutvid) we look at syncing audio and video automatically or manually in Premiere Pro.
If you record audio externally (and this is a good idea in many situations) there are a few ways of syncing the audio between the video file (which should have basic sound from the internal mic on your camera) and the sound file from your external recorder.
The manual way is pretty simple and straight forward but it does take time and if you are not careful you can make some mistakes. The basic idea is just to compare picks in your sound and sort of “superimpose” the audio file so it will match the sound print of the video file.
The other option is to select the audio and video clips in the project files bin by Cmd/Ctrl + clicking both files and right-click and choose “Merge Clips.” Choose a name and then choose the “Audio” option to allow Premiere to look at the audio waveforms of both soundtracks and line them up perfectly for you.