Trying to find the answer to how to use Miniflux with Reeder (app) and the Google Reader API proved to be a bit difficult due to the similarly of the app name and the API name. Or maybe I was sleep deprived. Either way, I finally worked it out but I never found a definitive "this is how you do it" just lots of vague Reddit threads so here we are.
In Miniflux go to Settings
> Integrations
> Google Reader
and set a username and password.
Once that's set, in Reeder choose "Reader" in the add account screen, set the domain you've got Miniflux hosted at and away you go.
To use Miniflux in NetNewsWire follow step one then choose FreshRSS in the add account screen. Make sure to not have a trailing slash after your domain in NetNewsWire otherwise it will error.
Update
As Trevor noted you must have HTTPS enabled on FreshRSS for this to work.
Using Miniflux with Reeder and NetNewsWire https://rknight.me/blog/using-miniflux-with-reeder-and-netnewswire/