1) Install the F-Droid app.
2) Open this link on your mobile device. (If you're using Firefox, long-click the link and choose Open with F-Droid App.)OR scan this QR code:
3) Choose F-Droid, then click Just Once.
4) When the Add new repository screen appears, click Add Mirror.
Alternatively: You can also remove the default F-Droid repo and add this mirror as the main entry point. This means that even things that currently can't be loaded from mirrors will be loaded form here. There seems to be an issue where some app versions are duplicated with this setup.
If you want to run your own mirror you can open an issue in the F-Droid admin tracker.