An XML sitemap is a list of pages of a website accessible to crawlers, also called search engine robots. There are several excellent online tools available to generate XML (and other) versions of sitemaps, for example xml-sitemaps.com. Typically the sitemaps are uploaded to the root of the website and by “submitting” the sitemaps to search engines, those search engines are notified of the existence of the sitemaps. However, one does not have access to the root of a Blogger blog to be able to upload a sitemap.xml file there, but the RSS feed of the blog, its atom.xml file, can take the place of the sitemap.xml file.
To create an XML sitemap for a Blogger blog proceed as follows:
Log in to Google Webmaster Tools with your Google account and add your Blogger blog to your list of websites.
Verify your Blog by adding the meta tag generated during the verification process, as instructed, to the blog template.
After your site has been verified go to “Add a Sitemap” in the Google Webmaster Tools. Add your blog’s atom feed URL at the Submit a Sitemap page.
The atom feed format is http://myblog.blogspot.com/atom.xml?orderby=updated.
Wait for some hours for Google to read your sitemap file and you will see the Sitemap Status as OK.
Note: The atom.xml contains only 25 posts by default and if you want to increase the number of URLs you need to change the URL to http://myblog.blogspot.com/atom.xml?redirect=false&start-index=1&max-results=100.
If you have hundreds or thousands of posts you have to add multiple sitemaps for your blog, like this :
http://myblog.blogspot.com/atom.xml?redirect=false&start-index=1&max-results=100 (for the first sitemap)
http://myblog.blogspot.com/atom.xml?redirect=false&start-index=101&max-results=200 (for the second sitemap) and so on.