First make sure your Android device is running version 4.4.2 or later. You can check this by going to Settings and clicking About. Your device must also be connected to the same network as your Roku. You can check the network your Roku is connected to in the network section of the settings menu.
To begin mirroring on a stock Android device, go to Settings, click Display, followed by Cast Screen. Then tap the Menu button in the top right corner of the screen and check the Enable Wireless Display box. Your Roku should now appear in the Cast Screen section.