If you have an iPhone or iPad, install the iOS Riot app from the Apple App Store.
If you have an Android device, install the Android Riot app from Google Play or F-droid.
Launch the app, and enable Use custom server options (advanced).
In the homeserver field, enter:
Note: You shouldn't need to modify the identity server field, which by default is set to https://vector.im.
Create a new account if you don't have one, or log in to your existing account.