Here is a video I created including most of the steps below = https://youtu.be/3APBQkbAcaA
download and install Reaper
download and install the free Youlean Loudness Meter plugin
add the stem tracks
Double click the ZIP file you downloaded to unzip it
Select all the tracks in the folder (ctrl/command + A) and drag all the tracks into Reaper's timeline
choose to add the files into "Separate Tracks" when asked
add a gain plugin (Reaper has one named "JS: Volume Adjustment") to one of the tracks and adjust so it rides around -18db and never peaks above -6db (this doesn't have to be perfect ... just get in the ball park)
in the mixer, drag the plugin to all the other tracks and it will copy the settings
now that the master out is not clipping, you can do other fine adjustments:
automate volume of each track where creatively compelling (here's a great video explaining how)
*adjust panning* to many of the tracks to give more space to all the mono files
[optional] If you want to learn more about mixing, here is a great resource for digging a little deeper.
When you are done, go to File > Render, render as the MP3 format.
make sure the file has your name on it
NOTE: I am above all determined to provide you with experience mixing audio *levels* correctly, so I'm primarily looking to see that the audio doesn't clip (distort) ... panning, EQ, and other filtering is icing on the cake (and you'll get better at this over the years as you gain more experience).