On OS X, download and run the LipikaIME Installer.

1. Open System Preferences -> Keyboard -> Input Sources
2. Click on the + button below the left pane
3. In the search bar at the bottom left, type lipika and click Add
4. Open LipikaApp and select your favorite transliteration scheme
5. Select LipikaIME from the Input Source list in the System Menu

For inbuilt schemes, refer to Transliteration Schemes

Alternatively, you can create your own custom key mapping using Google IME cannonical scheme format. You can edit any of the schemes from here and copy them into /Library/Input\ Methods/LipikaIME.app/Contents/Resources/Schemes/Google directory.