1) Go to www.screenr.com you can see a yellow button like in this image, press it.
2) After you have clicked it, it will ask you to run a java applet. Run it. Don't worry, it's not a driveby .
3) Adjust the dimensions of the box until you get your desired screen size. When you are ready to record click the red record . Then, a red box will indicate that you are recording.
4) When you are finished recording, press the DONE button just a little right from the original record button.