It's pretty easy assuming both of your devices are on the same network:

• install a game on your desktop PC via steam
• from your steam deck browse to the game in your library
• instead of Play/Install you click a little arrow next to the button and then select the name of your desktop PC
• the Play/Install button changes to say Stream, click that and it launches the game on your desktop and streams the video to your steam deck

if you have a good home network it is pretty much indistinguishable from playing it directly on the deck
I can play Elden Ring with no video artifacts or noticeable input lag

here is a guide which has some screenshots

https://www.pcmag.com/how-to/remote-play-how-to-stream-games-from-your-pc-to-the-steam-deck