See Flashing Guide
Run the following command in the zmk/app path
west build --pristine -b nice_nano_v2 -- -DSHIELD=blaster_pad -DZMK_EXTRA_MODULES="absolute/path/to/this/repo"
alternativly the -s /path/to/zmk/app
flag can be added to be run anywhere
west build --pristine -b nice_nano_v2 -- -DSHIELD=blaster_pad -DZMK_EXTRA_MODULES="absolute/path/to/this/repo" -s /path/to/zmk/app
the above command generates a file in zmk/app/build/zephyr/zmk.uf2