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 from here. (This is probably obsolete with newer F-Droid Client versions, but I haven't checked. It's still relevant for i.e. F-Droid Classic though)
Read the docs, if you want to run your own mirror.