In this tutorial, I would talk about the low end of our mixes and how to use a reference track to get it in the right ballpark.
Low frequencies are usually related to two instruments: kick and bass. This range of frequencies is hard to manage because of two main factors. First, from the acoustic point of view, it is hard to tame the room resonances and phase cancellations in that range. Second, human ears are not very sensitive in that range. Humans like bass frequencies and an inexperienced mixing engineer might tend to over-boost in that range. Let's see how we can use a reference track to mix low frequencies.
Some helpful advice
Use a good pair of headphones to double-check the low end. (See also the article MIXING WITH HEADPHONES: 4 TOP TIPS)
Use an LPF for other instruments to create more space for the kick and the bass guitar.
Use one or more reference tracks to make sure your low-end is in the right ballpark
How to use a reference track to mix low frequencies
Import the reference track in your project. I suggest importing two or three reference tracks.
Volume matches the reference track with your mix. To do it, you can simply reduce the volume of the reference by turning down its fader until it matches the volume of your mix. You can use an RMS or better an LUFS meter to facilitate this step. (See also the article DB AND METERS...WHO ARE THOSE STRANGERS?). Matching the volumes is very important to avoid being fooled by the difference in volume.
Check if the low end of your track is in the same ballpark as the reference/s. One way to do it is to put an LPF on the master bus and cut out everything but the low end so you can be focused only on lower frequencies. Your mix doesn't need to perfectly match the reference as every song has its characteristics, but it has to be in a reasonable ballpark.
N.B. Don't use low-quality Mp3s as reference tracks. The Mp3 format can cut many frequencies to reduce its size. Train yourself to listen to high-quality music or your internal reference will be misaligned compared to the rest of the pros.