• Go to Extensions → Template Manager → (J15) rt_momentum_j15 → Settings; or (J17) rt_momentum → Show Presets
• Configure the Settings
• Click (J15) Save Custom Presets as New; (J17) Save Preset
• Follow the Preset Saver procedure

You can edit the prebuilt presets in the gantry.config.php file, or use the User Interface method outlined above.