How to set the URL for Blogger posts? Custom URL for blog post

URLs and Blog Posts

When you first publish a post in blogger, an URL (called a permalink in blogger) is automatically generated for that post. It looks like:

Years ago, Blogger chose the WORDS-ABOUT-YOUR-MY based on the title, or the first words in the post if the title was blank. They used some rules eg leaving out "the" and other common words, and putting numbers on the end so that every post has a unique URL (called a "permalink" in Blogger).

However Blogger have now provided a tool that lets you choose the WORDS-ABOUT-YOUR-POST separately from the post-title.

How to change the customisable part of the URL for a post

Edit the post in the usual way.

In the Post Settings area (currently at the right hand side of the editor), there is a section called Permalink.
Click on Links to show the options in it.
Click the custom URL radio button
Type the words that you want to use in WORDS-ABOUT-YOUR-POST into the Custom URL box
Click Done.
Finish the post, and Publish it.

Restrictions
The only characters you can use are:

lowercase letters (ie a, b, c ... z)
uppercase letters (ie A, B, C ... Z)
digits (ie 0, 1, 2 ... 9)
underscore (ie _)
dash (ie - )
full-stop, also known as a period (ie .)

It looks like there is no restriction on the number of characters you can put into the URL. For example, I was just able to make a post in my test blog, with this URL:
http://bhat-draftarticlestore.blogspot.ie/2012/07/123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-123456789-.html
(you cannot see the post, because that particular blog isn't open for public reading.)

If the combinaton of yyyy-mm from post-date (which you can change - see Setting the Post Date) and WORDS-ABOUT-YOUR-BLOG is not unique, Blogger will leave out the last character(s), and put in numbers to make it unique.

It only applies to Posts, not Pages: the only way to influence the URL / permalink for a Page on your blog is to choose the initial words in the page-title very carefully. (Ref: the difference between Posts and Pages)

Changing the custom-URL words after publication
Originally, after you hit the Publish button for the first time, there was no way change the permalink: if you click on the Permalink option in Post Settings, you are shown the custom value that you chose, but you cannot change it.

Changing the custom-URL words after publication
Originally, after you hit the Publish button for the first time, there was no way change the permalink: if you click on the Permalink option in Post Settings, you are shown the custom value that you chose, but you cannot change it.

However you can now:

Edit the post.
Click the Revert to draft button.
Edit the post URL in the same way

and the URL of your post will be changed. Note that if you do this, the post characteristics (view count, comments) are kept. This means that Blogger must be associating them with the unchanged internal post-identifier, not the URL.