Give yourself at least a few hours per song to get a good mix. You'll be listening to the song many times, both as a whole recording and as separate tracks. You also want to leave the final mix for a few days or a week to rest your ears before you listen for a final time. In the end, the amount of time you'll need to mix a song depends on the number of tracks recorded, your level of experience and the point at which you or the artist sign off on the mix.
Make sure that all tracks are recorded well (i.e., they sound the way you or the artist intended) and that they are accessible. If there are any glitches, bad notes or other problems, fix them either by re-recording or with the help of software. Not everything can be fixed in the mix. Wait a few days after the final recording session before you begin the mix so you can approach the task with fresh ears.
Listen to the entire recording, all tracks at once, several times. Note which parts should be up front and which should be in the background. Also note where instruments should be panned, meaning where they are placed in the left-to-right sonic space.
Conquer the most obvious tasks first. If a bass is too boomy, reduce the lowest bass frequency. If some vocals are too shrill, reduce the higher frequencies. These simple steps may solve your problems; if not, you can do some fine-tuning later.
Not every instrument or vocal should be in the same sonic space. Lead vocals should be up front (i.e., louder) than background vocals. Lead guitar should probably be louder and panned to the side, whereas drums should be panned to the center.
Sometimes, the bass drum will conflict with the bass guitar in certain frequencies. This is where you should carve out space for both instruments by raising or lowering frequencies. Listen to the tracks through speakers as well as through headphones to get different perspectives. Listen to the entire final mix on different speaker systems (e.g., a living room system, a car stereo, a boom-box).