Just copy the .keylayout and .icns file into /Library/Keyboard\ Layouts/ on your Mac! You probably need to reboot and then you may choose the layout under System Preferences->Keyboard->Input Sources.
After the above change, you'll see the Chakma Layout in the menus like below:
This is how the keylayout looks like:
Feel the need to update the layout to your liking by adding in the Chakma language characters? Download the keyboard layout tool Ukelele and play with it.