From the "SHIELD Hub" row of the Home screen,
launch 
to access GeForce
NOW for immediate access to more than 50 popular PC games
from NVIDIA cloud gaming supercomputers to your SHIELD at up to 1080p
video resolution and 60 frames per second. You can even purchase
newly released games from the store.
NOTE
To stream games at 720p resolution and 30 frames per second, you
need a broadband minimum download speed of 5 Mbps (10 Mbps recommended).
To stream games at 1080p resolution and 60 frames per second, you
need a broadband minimum download speed of 30 Mbps (50 Mbps recommended).
Ethernet connectivity is recommended for the best experience.
NOTE
Navigate through Geforce NOW using
the D-pad or left thumbstick on the game controller.
When you launch NVIDIA GeForce NOW for the first time, you will be prompted to log in. You have two options for logging in: (1) By creating an NVIDIA account or (2) Logging in with the Gmail account that is associated with your SHIELD.
Settings lets you edit GeForce NOW account information, test your broadband connection and specify preferred Frame rate and Max resolution in "Streaming quality."
On-screen controls provide users with additional
controls necessary to navigate some GeForce NOW games. To access
the on-screen controls, long-press the Start 
button.
The on-screen controls will bring up a menu screen with three options: Keyboard, Quit, and three-dot menu.
If you need to input information into a dialog, use the "Keyboard" selection. If you want to quit the game, select "Quit game."
Most of the navigation options are straightforward. Clicking the three-dot menu > Help will display the full list of available functions.