Add effects to your tracks as you see fit. There are no rules, but generally add light compression and equalization to each instrument. Use reverb sparingly to give spaciousness, but be careful not to add so much that the tracks get muddled together. Adjust the frequencies of each instrument so that each holds a separate area of the tonal spectrum. Too much middle range across several instruments, for example, will make for an unclear mix.
Center-pan the bass guitar and drum. These instruments, along with lead vocals, should be the only tracks panned to the center. Mute the other tracks for now.
Un-mute the drum tracks and mix them with the basses. When setting pan, position the drums as if hearing a live drummer, with toms moving from slightly right to left, the snare to the right, and cymbals ranging across the spectrum. Be sure that the cymbals' volume is not overpowering; this would interfere with later mixing.
Add in the background instruments, like strings, synthesizers and rhythm guitars. Once again, pan like you were listening to a live band onstage. Guitars can be set to about 30 left and right, with synthesizers and strings slightly wider. Again, be careful with the volume. Each instrument should be heard clearly (though guitars will become one sound spectrum) but should not be overpowering.
Add lead guitars or any other solo instruments, like piano or brass. Often, a lead guitar solo will be panned near 30 right (don't overlap the rhythm guitar, though), but feel free to experiment if the melody allows for it. Volume can be used slightly more to bring out the melody and/or harmonies.
Insert the vocals, with the lead track panned at or near center and any harmonies within about 10 degrees left and right. The vocals need to be loud enough to clearly hear the words, but be careful not to overcompensate with volume. Keep harmony vocals more to the background.
Listen to the song in its entirety both with your headphones and with outboard speakers, if available. If you notice anything that does not sound good, go back and fix it.