Did I not post the directions somewhere in this thread? I definitely put it in the YouTube comments months ago. Please note, these aren't my instructions, someone on the old CTGP-cafe discord had figured this out:
All you need to do to get this to work on Cemu is:
1. Install and enable CTGP Cafe
2. Download and extract MK8 Ultimate
3. Navigate to cemu\graphicPacks\Mario Kart 8\Mods and create a folder called Ultimate
4. Inside Ultimate create a file called rules.txt and give it the following contents
[Definition]
titleIds = 000500001010ec00,000500001010ed00,000500001010eb00
name = Mario Kart 8 Ultimate
path = "Mario Kart 8/Mods/Ultimate"
description = Enables custom UI and 104 custom tracks
version = 3
5. Move the content folder from Mario Kart 8 Ultimate\mk8uloader\vol\000500001010EC00 to your graphics pack
6. Now inside the content folder, cut the course folder and paste it in MK8's update folder
7. In Cemu, right click on MK8, click "Edit Graphics Packs" and enable "Ultimate"
People have said this works. So hopefully this gets the job done for you.