In order to use them you must use the Steam beta client.
The Steam platform continues to open up and expand its borders. This time he has achieved something that many fans will have caught by surprise. valve has added support for the Joy-Con from Nintendo Switch to Steam, so you can now play the various games with the small controllers, according to an update on the Steam blog.
To use the Joy-Con you must access the Steam beta clientAt the moment this is only possible if you have the steam beta client. You can easily get it by following the steps on the official Steam beta client website. In addition, it has improved compatibility with classic Nintendo Online controllers.

The Steam post states the following in reference to the Joy-Con: “Added Joy-Con controller compatibility Nintendo, both individually as a mini-gamepad and combined in pairs”. In this way we can play with either one or both Joy-Con.
Last month Valve already reported that it had given support to the classic controls of Nintendo Online. These are: NES, SNES, Nintendo 64 and SEGA Mega Drive and Genesis. Now the latest addition is the Joy-Con, which despite the fact that they may have drift is great news to increase the possibilities of how to play.
More about: Nintendo Switch, Joy-con and Steam.