I did this while following along with a video tutorial at Comic Book Graphic Design. This is a great site for practicing drawing with fully narrated, step by step videos. As a bonus, the content fit with the themes in Frequency Array pretty well.
I started out using pencil and then finished up with gray-scale markers. Small color details have been done in marker as well.
If you want to view more sketches and stuff related to Frequency Array, please check out my Patreon site.