The process was: launch sc-pageant.exe, insert smartcard in reader, and ready to connect with any SSH client with agent support.
However it is not open source and you have to register your key if you don't want to get an annoying popup from time to time (iirc it was 1€, but you could also email the developer if you were using an OpenPGP card).
But the licensing is meh. 20 euros for one public key hardcoded to the license...
I'm no expert on this so I may have missed something.
Except I remember ending up with multiple copies of gpg-agent running, for no obvious reason (which had to be killed before I could start a session - hmm. Maybe I remember finding this solution annoying). I think support for PIN-entry had its own annoyances.
[Note: I haven't tried this at all recently - I just did a quick search to try to refresh my crumbling memory]
I know the most popular use of Yubikey + SSH is via OpenPGP but it involves a lot of things which are not working very well on Windows (namely GPG4Win, unstable gpg-connect-agent, etc.).
With this fork, you just need to install the official Yubikey mini driver to have it work with PuTTY CAC. (You will also need Yubikey Manager to configure your key).
Connect your Yubikey, launch PuTTY CAC, look in SSH>Certificate, enable Attempt cert auth, Set CAPI cert (choose your Yubikey cert).
It even works through Windows Remote Desktop.
https://github.com/NoMoreFood/putty-cac