Thumbnails from videos

THESE are the directions to create a thumbnail of my post. I am having a hard time understanding where and what post-img goes….

To define thumbnails for each post, you must use the custom field tag:
“post-img”, followed by the image url as value.
Example:
Custom Field: post-img
Value: /thumbnails/picture1.jpg (location of image file on server)

If this is the video I want to post where do I put the above directions? http://www.youtube.com/watch?v=GdfRg0rYZxU&feature=feedf

What about creating a thumbnail for a word only post?

Thanks!