Doubling is usually done by making a copy of one track, panning those two identical tracks hard left and hard right, and adding a slight delay(10-30ms) to one of those tracks. The delay between the two tracks creates the stereo image.
You can achieve this slight delay in two different ways in your own recordings.
Either you use a plug in that will provide an actual delay, or you "nudge"(move) one of the audio tracks by a user-determined amount of time(not all software can do this) relative to the original one. Moving one of the tracks will create the time delay, and thus the stereo image.
Personally I prefer the latter method since it doesn't require any extra processing power like plug ins do.

Another approach to getting a big/wide stereo sound is simply recording the exact same piece twice, onto two seperate tracks, one panned left, the other to the right. Overall I prefer this method, but it does require some very consistent playing in order to get a good result.