1. In the Camera Editor, select the Anamorphic predefined film back, or create your own film back.
2. In the Image File Output section of the Render Globals window, set the X Resolution and Y Resolution to values appropriate for an anamorphic lens (for example, 500 and 423), and set the Pixel Aspect Ratio to an appropriate value (for example, 2). The Pixel Aspect Ratio (under Render Globals) produces the same effect as the Squeeze Ratio (in the Camera Editor).
3. Render your scene.

View the image file. Note that this image file will be distorted.