Creating custom permalinks on Blogger is easier than you might think - but is far from elegant.
The URL of each post you publish is based on the Title of your post. However, if you later change the Title, the URL stays the same.
So to create a custom permalink, just make sure you initially give your post the same Title as you want to use in its URL.
Once the URL is created and fixed, you can edit the post and change the Title to something more catchy - its address on the web will stay the same.
It's not a way of going back to change permalinks of previous posts, but it's a sneaky little workaround for Blogger users who want control of the permalinks for their future blog updates.
NB Check the URL of this post and you'll see I originally went with 'how to create custom permalinks on Blogger' for SEO purposes. After posting, I changed to the text you see above. Watch out for the URL truncating, like this one did, if you use a lengthy Title to begin with.
You can argue with me on Twitter about whether it harms your SEO to have a different Title and URL - personally I think, at least in some cases, it's more valuable to have full control over each variable separately, than have them forced to match.