1. Pan all instruments to the right and left. Make sure not to pan your kick, bass guitar, or vocals, keep them in the middle.
2. Look for the loudest and quietest sections of the song and make 2 loop markers.
3. Take all your instrument faders and bring them down.
4. After, the loudest part make sure to set the vocal fader so that the level is around -20 BFS (if you're using the K-20 setup for mixing).
5. Make sure you click your output from Stereo to Mono. For this you can use a plug-in, a stereo fader panning, or your console mono button.
6. Select all the vocals and turn them to a 'can just make it out' level and now bring up the rest faders so that you can hear each instrument in the track. Make sure you do this for the two loops we made earlier and adjust to find a good balance.
7. Turn the volume up to your usual level and click back from mono to stereo.
Now basically you should have a decent balanced mix. :)
You can tap it up with some automation later if fader levels make one instrument too loud or too soft.