"What are the most common ways to convert MIDI files to .MOD files? Or create .MOD files?"

First you need an editor or tracker that will create .MOD files, such as PlayerPRO 5.8 for Mac, or MODPlug Tracker 1.2 (both freeware). After that, it's quite simple, and the procedure will probably seem very similar to using any modern audio editing software. You can construct files from scratch, composing MIDI sequences and incorporating the use of samples within, or if you already have MIDI files you'd like to augment with samples, the MIDI components can be used as the basis for a .MOD file (most editors will allow you to drag or import the MIDI file right in), and from there you add the samples necessary, and the tracker exports the .MOD file for you.

Of course, this means you have to have your audio samples ready if you don't yet...and that's another day's inSync.