Navigate to the Home screen on the Roku. On your PC, open Chrome and go to Netflix or Youtube and start a video playing. In the upper right-hand corner of the browser, click on the cast icon. That will bring up a list of available devices to cast to, the Roku should be in that list, if so, select it and the video will appear on the TV connected to the Roku. If the Roku doesn't show up in the list, it could be that your model is too old (maybe check with Roku tech support), or both need to be connected to the same network or some other network issue.