How to create a shape in the UV image editor for using as a mask in VSE? I'm using Blender for editing desktop recordings that I make using Open Broadcaster software.

I would like to highlight areas of the screen to draw attention to them by surrounding them with a red rectangular box.

I've created a previous post that asks: How to highlight areas of a desktop recording in video sequence editor

The answer to that post is fantastic, and I would like to now understand how to create an image in the UV editor for using as the mask. How can I do this?

I'm a complete Blender noob and have not used any part of Blender except for the VSE.
What you want to do is split a screen and open a UV/Image Editor, or just open one up through the bottom, lefthand icon:



Load in an image with the Open button (you can screenshot a frame of your video) so that you know where to put your mask.



Still in the footer of the UV/Image Editor, add switch from View mode to Mask mode.

 --> 

Add a new mask:



Now there are three keys to remember:


Ctrl +  LMB adds a Control Point where you click.
Once you have three or more points, use A to select all/deselect all.
If every point is selected (yellow), use Alt + C to connect the points.