When I got the Steam Deck last year there were a ton of steps involved in installing GeForce Now. However, the process has now changed and it is much easier to install GeForce Now on Steam Deck and Steam Deck OLED.
To get started, download a browser on your Steam Deck. You can use Chrome, Edge, Firefox, or any other available browser.
To install GeForce Now you need to be in Steam Deck Desktop mode.
Step #1: Download GeForce Now Installer
Open your browser and go to: https://www.nvidia.com/en-us/geforce-now/download/ to download the GeForce Now installer. When you’re on the page, scroll down to the footer section and click on “Get Started” under the Steam Deck (BETA) tab.
Click on download when prompted with the following message:

Step #2: Install GeForce Now On Steam
With the GeForce Now installer is downloaded, go to the downloads folder and right click on the “GeForceNow Setup” folder. Scroll all the way down to the “extract” option and click on “extract archive here” from the sub menu.

Now click on the extract folder, also named “GeForceNow Setup.”

Finally, open the “GeForceNow setup” again. So, it is GeForceNow setup > GeForceNow setup; both folders have the same name for some reason.

A small window will pop up; select on “Execute” to continue the installation process.

The final step is to select “yes” on the next window to trigger the installation of GeForce Now on Steam Deck.
You will be asked to go to back Gaming Mode to start using GeForce Now. The app will automatically be added to Steam.
When back in Gaming Mode, navigate to “non Steam Games” tab in Steam where you will find GeForce Now ready to use.

GeForce Now Setup
From the top left corner, we are going open the menu and go to Settings. There, the first thing we will do is limit the frame rate to 60FPS.
We will keep the resolution at 1280×800 and bit rate at 25. If you have Decky Loader installed, you can add custom artworks to GeForce Now because as you can see the cards don’t have any artwork attached to them.
But this is completely optional.