To many it may seem easy, but to me it sounds like a big issue. The main problem I am thinking of is syncing. If I recorded myself playing the guitar and took the music and added to the video would there not be a delay or sync problem? I am sorry if this question lacks detail. This is how confused on how to do this. If I were to buy something could it be cheap and efficient?
This might be old school, but you have to use a clapper.
Start your audio recording and your video recording.
Go in front of the camera and clap your hands together so this image is recorded on film and on your audio recording.
Then simply play through the song.
You now have a video that's starts with your hands clapping.
Later you can do a better recording using the original audio as a "click track." As long as you save the audio of the hands clapping and the film image of the hands coming together, you should be able to add a multi-tracked audio to the film image and have it synced up.
Later, you can edit out the hand clapping.
Hope that wasn't confusing?