One sticky point that took me a bit to clue in to was you need to login to an x11 session not the default Wayland. Unless sunshine has been updated to support wayland, this was about Feb I discovered that.

To change just log out and select x11 in bottom left corner of the login screen. Dont forget to switch back next time if you arent using sunshine

Providing more specific information about what isn’t working would help resolve the issue.

The flathub page lists some additional steps that need to be performed to allow virtual input. Did you do this?

The description of the app (when you open it in Software or on Flathub through your browser) tells you what to do to make it work