Before heading underground, you must activate the data pack inside the specific world save where you plan to play, alongside both the Cobblemon and Deeper and Darker mods.
Enabling on a New World
1. From the singleplayer menu, start creating a new world and click the Data Packs button on the setup screen.
2. Click Open Pack Folder to open the temporary folder for your new world, then copy the Otherside Cobblemon Spawns Data Pack archive into it.
3. Return to the game window, hover over the pack in the Available column, and click the arrow to move it into the Selected column.
4. Click Done and finish creating your world.
Adding to an Existing World
1. Select your world in the singleplayer list, click Edit, and choose Open World Folder.
2. Open the datapacks subfolder inside your save directory and place the data pack file inside it.
3. Load into your world. If commands are enabled, open chat and run /datapack list to confirm the pack is active, or run /reload if you moved the file while the world was already open. If commands are disabled, simply save, quit to the title screen, and reopen the world so the spawn rules load before you enter the Otherside.