Checkout Fate Client!
1. Download one of the capes below.
2. Download one of the cosmetics in the Cosmetics section.
3. Open the vanilla folder and delete the cape.png in the folder (If in this folder no png then the cosmetic is not compatible with a cape)
4. copy your cape.png and paste it in the vanilla folder.
(If you can't see your cape. you properly have to include the cape open your skins.json and add "cape": "cape.png", like you can see here)