1. Download the mod for your version https://www.curseforge.com/minecraft/mc-mods/vtubruh-lotr-mobs-new/files
2. Download additional files https://cdn.modrinth.com/data/pGnVL7c7/versions/KrT3mRRH/vtubruhlotrmobs_map_files.zip
3. Download Geckolib for your version https://modrinth.com/mod/geckolib/versions
4. Create a world and exit it
5. Go to your Minecraft folder, then to your world folder, for example .minecraft\saves\world
6. In the world folder, there will be a dimensions folder - put the downloaded vtubruhlotrmobs folder there, replacing the old one (if prompted)

In this case, all you have to do is close Minecraft and follow the instructions above. 
If after re-entering you find yourself underground, just jump.

Why is this happening? It's related to the large map, which I haven't been able to load automatically in the dimension yet. 
Hopefully, I will fix this in the future.