To use Screencast-O-Matic, simply download recorder launcher by clicking here and then drag a box around the area which you wish to record. Click the red "record" button to begin. You can choose whether to add narration, or just show steps to something.
Upload to YouTube or Vimeo
To use either, you'll first need to create an account (both have free plans!). With YouTube, you'll see an "upload" button in the top right corner. Click it and then either drag and drop your file or click to select a file that's saved on your desktop.
Once your video is uploaded on either platform, you'll see buttons that say "share." Click this and you will see a URL -- copy it.