One little-known feature of the GNOME desktop environment is that it has the ability to record screencasts. By default, the keyboard shortcut to do so is CTRLALTSHIFTR. Press it again to stop recording. This will save the video as a webm video into your Videos folder.
There is only one problem with this method: by default, it stops after 30 seconds. To fix this, open a terminal and type the following command
gsettings set org.gnome.settings-daemon.plugins.media-keys max-screencast-length seconds
seconds to the number of seconds until it stops automatically. Remember that you can always stop it manually.
While the video is recording, there will be a recording icon to the right of your top bar.