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 different games with the small controls, depending on an update from 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.