The best (most accurate) way is to adjust the levels by ear before you mix the tracks.
- Import the tracks (they will appear one above the other in the Audacity project)
- Select all of the tracks (Command + A) and apply the Normalize effect. Normalize to 0 dB. This will make all tracks as loud as possible without distortion.
- Use the track “solo” buttons to listen to one track at a time and find the quietest track. (at this point I find it convenient to move this quietest track to the top - see http://manual.audacityteam.org/man/audio_track_dropdown_menu.html)
- Compare another track with the quietest track and use the track Gain control (http://manual.audacityteam.org/man/audio_tracks.html#gain) to reduce the volume of the louder track so that it matches the quieter track.
- Repeat step 4 for all the other tracks.
For a semi-automated method, see this topic: https://forum.audacityteam.org/t/replaygain/22589/1