https://gitgud.io/argonvile/monster-mind/
There is actually a detailed guide at gitgud.io! Under Section 4 "Adding your own Pokemon" it describes all of the steps necessary to add your pokemon. But really, step 1 is getting to where you can build the game, so start with the "Workstation Setup" page in the Wiki to install all of the HaxeFlixel libraries, VSCode, and about 50-100 other little things ha ha.
It will be an adventure! But I wish you the best of luck, it would be very cool to see more Pokemon.