Microsoft has opened registration for the Project xCloud preview. Sign-ups are available here.

Here is an overview of the sign-up process, via Xbox.com:

Register for Project xCloud (Preview) – You will need a Microsoft account to register. If you don’t have one, you can create an account for free during registration.
We’ll email you with next steps – If you are chosen to begin the technical preview, we will send you an acceptance email with next steps. The first round of the technical preview starts October 2019. We will only be accepting a limited number of participants in the initial test, but will admit new players throughout the year.
Get the Xbox Game Streaming (Preview) app – Once you get accepted, install the app to your Android device, then go to the Project xCloud section of the app and log in with the account you used to register for the preview.
Pair your Xbox Wireless Controller – Follow the steps in the app to pair your Xbox Wireless Controller with Bluetooth and Android device. Optionally, you can choose to clip the controller to your phone.
Go play and share your feedback with us – Select any game in the cloud library and start playing on your Android device. Provide feedback in the app to shape the future of game streaming.

And here is an overview of the Project xCloud preview itself:

Stream Xbox games from the cloud
Get a first look at game streaming with Xbox in Project xCloud (Preview). Play Xbox favorites on your Android phone or tablet directly from the cloud over Wi-Fi or your mobile network. As we’re starting to roll out this test, we invite you to help shape the future of game streaming.
Play console-quality games
Test streaming console-quality titles from the cloud—no waiting for downloads. During the preview, you don’t need to own games or a console. We’ll be adding more games over time for you to discover and play.