Install the latest version of Fabric (https://fabricmc.net) or Use Multimc to create a new instance with Fabric:
- Install MultiMC
- Add/Create a new instance
- Select 1.21, click ok
- Select said new instance, Edit Instance, go to Version tab
- Click on the right to Install Fabric, select the latest version (or 0.15.11 at this time of writing), and click ok.