OK, here's a very simple description of what I do. This can be done in any audio editor like Audacity, Adobe Audition, Cool Edit, etc. This process needs only one pair of stereo tracks (one L and one R) so you don't even need a multi-track program, just stereo. There are many steps, so I will suggest file names. If you use the same file names, it might make it easier to keep track of the process.
1.) Start with your mono file. Make a 2-track copy. Both tracks (L and R) will have exactly the same audio. Peaks should be no louder than -6dBFS. Save a copy of this, named "01-stereo-raw.wav" assuming we are working with .wav files. In addition to the copy you just saved, keep the audio open in your software.
2.) Now, using the file which is still open, invert the polarity of only the right channel. Save a copy of this new stereo file, named "02-stereo-antiphase.wav" In addition to the file you've just saved, copy this audio onto your clipboard.
3.) Now we'll make some test files with different versions of a widened image. If "01-stereo-raw.wav" is not still open (from step 1), then open it again. We will paste the "02-stereo-antiphase" audio (which is in your clipboard since step 2) on top of the "01-stereo-raw" audio. However, don't just paste the clipboard in the same place and the same level! (If you did that, the level of the L channel would become 6dB louder, and the R channel would disappear.)
For our first test, paste the clipboard onto the raw audio, in such a way that the raw audio and clipboard audio are combined (added together). You do NOT want the clipboard audio to replace the raw audio. The beginning of the clipboard audio should start 25mSec after the beginning of the "01-stereo-raw" file. The level of the clipboard audio should be 50% (or -6dB) compared to the original clipboard level. Save a copy of the resulting audio as "03-stereo-test.wav" and listen to it. Keep written notes so you have a record of what delay and what level you used on each test file.
How do you like the test? Do you hear the widening? You can try other settings. There are no "correct" settings. Settings that sound good with voice might be very different from settings that sound good with a bass line. If you make the pasted level a lower percentage, you will hear less widening. If you make the pasted level a higher percentage, you will hear more widening, but it might start to sound a bit strange. If you make the delay time greater than 25mSec, it will eventually start to sound like an echo. Try some different tests. Be sure you always go back to "01-stereo-raw.wav" as your main file, and add the "02-stereo-antiphase" as your secondary (lower level) file. Save each test with a new file name (e.g. "04-stereo-test.wav" etc.), and be sure to make a note of the changed settings.
The above process "widens" the stereo image, but does not necessarily make it "fuller." Many audio editing programs can generate artificial reverb which will add some "fullness" or "depth" to the sound. Once you have a widened stereo image, adding a tiny bit of stereo reverb may be helpful.
Remember that human hearing does *not* perceive directionality very well at low frequencies. So a given amount of widening (a given delay time and level) might sound very noticeable at higher frequencies, but might be barely noticeable at lower frequencies.
Good luck! Let me know how this turns out, or if you have any further questions.