Listen to the audio all the way through after everything has been recorded. Don't move any of the faders (volume levels) yet, keep them at the same level as during recording. Take notes on the what parts sound like they should be brought out in the mix (too quiet, need more emphasis) and which parts should be buried more (glaringly loud or strident). Also note any problem areas in pitch and rhythm.
Isolate parts that connect together musically and play them by themselves. For example, if the guitar and the vocal tracks were recorded on different microphones, hear how they sound when played separately from the other instruments. Do they blend well? Raise and lower the respective volumes to allow both instruments to work harmoniously. When finished, bring them back into the mix and hear how they all work together. Adjust other tracks to fit the volumes of the leads.
Listen to the vocals, if any, and fix any wrong pitches using a pitch shifter tool (this comes with many digital audio editors). Be aware that shifting pitches can sometimes give a track a "too perfect" or computerized sound. If a pitch is seriously below or above the intended note, it might be better to re-record rather than make an obvious edit.
Play, replay, and re-replay the audio to asses the mix. By the end of the mixing process, you should have heard the piece dozens of times.
Export the audio from the editing program as a high-quality audio file (24 bit 96kHz wav or aiff format) into a mastering program such as Sound Forge on OC or Sound Track Pro for Mac. Many audio editing programs, such as Sonar and Cubase, can do mastering in program and do not require any exporting. Bring in another set of ears at the end of the mixing process and the beginning of the mastering process. At this point you have heard the music so many times that you may be unable to detect the nuances in the audio as effectively as when you started.
Do the mastering in a separate room if possible, and one that is designed for mastering (perfect acoustics, no echo, very little furniture). Because you're going for overall sound in mastering, it's important to ensure that external factors don't affect the playback of the music.
Normalize the audio by bringing down the highest highs and lowest lows. These should be subtle changes that seek to keep the track even. Consider running a compressor on the audio, which can automatically rein in the peak frequencies. Too much compression, however, can make the audio sound flat and artificial. Remember, its a delicate balance. If you want your songs to fade out, now is the time to do it.
Consider getting the mastering professionally done. Even if you are quite capable with the program, specialized rooms and high-quality monitors (studio speakers) make more of a difference in the mastering process than any technical wizardry.