To create your own URL for a specific post, simply click "permalink" and select "Custom URL", and enter your customURL in the field given. If you wrote the post in July of 2012, your new URL will look like this:
http://YOURBLOG.blogspot.com/2012/07/customURL.html
The red colored bolded area is the portion of the URL that is customizable.
If the custom permalink you entered already exists, Blogger will attempt find a free one for you.
At present, the characters allowed in a custom URL are limited to: a-z, A-Z, 0-1. The only special characters available are underscore, dash, and period.