Others will probably have different approaches. For me, quick and dirty would be like this: on a white background in Photoshop, PaintShop Pro, etc., draw in the outline of the interior shape — that is, the shape to be occupied by the video. Select a fill color– red, for example — and fill the interior shape. Save this drawing with red as a transparant color. Open this saved drawing in your editing software and run the video behind it.
In a program like Vegas, which I use, I would put the drawing on track one and the video below it on track two. The video will show in the transparant portion of the drawing. Use the Pan/Crop tool to adjust both tracks as necessary.
