Inside your created Channel click +New Video and then click Screen Recording which will appear below.
A blue button will appear in the bottom middle of your screen that reads Start Recording - click this button.
Your browser will then show a pop up asking you to share your entire screen, choose Entire Screen and click Share on the bottom right.
Your recording will immediately start and it will also record your microphone so you can speak. When you have finished click Stop Recording which is a red button that shows in the middle of your screen near the bottom.
Once you click Stop Recording the pop-up will stay and you can watch your recording back. If you would like to start again click Close and start again.
If you are happy with your screen recording, enter a name in the field below your video and then click Upload and Continue - this will automatically attach your screen recording to the Channel you are currently on.