qmk firmware really wants you to have their repo in your home directory; a symlink works and is much handier, ie qmk_firmware -> Source/qmk_firmware.
Then in there, you'll need to add a symlink to the directory containing your keymap; in this case: ~/Source/qmk_firmware/keyboards/v60_type_r/keymaps/starry-night-custom -> ~/Source/qmk-keyboard-layout/starry-night-custom.
When you want to build your firmware, you can run:
qmk compile --keyboard v60_type_r --keymap starry-night-custom
from anywhere, which will drop a new v60_type_r_starry-night-custom.hex in the root of ~/qmk_firmware, which you can flash your keyboard with.
You'll need files from Massdrop's Github releases page; the platform-specific mdloader file and applet-flash-samd51j18a.bin for some reason.
To build a new Drop Alt file:
qmk compile --keyboard massdrop/alt --keymap paul
Then to load it in the keyboard:
$ cd ~/bin/massdrop
./mdloader_mac --first --download ~/qmk_firmware/massdrop_alt_paul.bin --restart