Recording acoustic guitar is a bigger problem than electric. If you have read my tutorials on electric guitar recording you can really just use an sm57 microphone, spend some time positioning the mic, get the tone you want from your amp, then press record. However, recording acoustic guitar definitely creates some additional problems.
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First problem is the microphone. Usually a dynamic microphone is not really well suited to this. However I have gotten some pretty good sounds using an old Electro Voice PL80 especially when coupled with a cloudlifter. But a cloudlifter costs about as much as a cheap large diaphragm microphone, so I would go for the mic instead. Besides you can use it for the vocals later. Since we are doing this for cheap try to not spend more than $150.00.
Most people use the cordoid setting for guitar or vocals, but since this is very low budget, and yet we want the biggest fullest sound - you have to try the omni setting, but do it with some sense. Try different rooms in your house to record the part. You might have to play samples of the part many times facing different ways in a room, but it will be worth it if you can find a good area where sound is reflected around in just the right way. In desperation you can even try the bathroom, which will give you lots of reverb. Don't be afraid to take your time and experiment, remember every time you go over the part it is going to get better and better. Also remember the part must be tracked to metronome or some kind of percussion track generally I use a live drum set.
Now just as with electric guitar you must double track the acoustic guitar, however this needs to be approached in a more careful fashion. First, if you are doing a strumming part, try to get a lightly different sound by using a different room, changing the setting on the mic, or move the mic closer a bit or further away. Be careful though of the proximity issue though, this can help or hurt and sometimes mere inches can be the difference between a good sound and a great sound. Make sure you play the part exactly like the first part!
Second keep the track with the most mid tones to come right down the middle, while the track with the most highs should be split with maybe no more than a 12 ms delay, eight would probably be better. You can also run similar sounds panned one side to the left and one side to the right to fill out a part in the mix. If the main part is a finger-picked part then the "double" should be a different part, less busy, and played in a higher register. Even single notes inside the chord works well and will fill out the sound considerably without taking away from the finger-picked structure that you have taken great pains to create. Remember the key is to get the fullest sound possible while using the fewest instruments as possible so that your tracks can "breath" and have "space" for the vocals.