I don't download many modules and when I do, I forget how...
So I just in case I might forget it again and who knows it might help you out too!

To install a module you need the github link of the module you need.
Once you have that, follow these steps:

1. SSH login into your MagicMirror
2. Navigate to tyour Magicmirror modules directory, might work with cd MagicMirror/modules depending on your starting path.
3. Now type  in git clone (link here)

This should download the module to your MagicMirror instance.
Configuring your module should be mostly the same as any other module by adding it to the config.js file, any other configuration should be listed on the github page of the module you downloaded.