How to Add a Google Sitemap to your Blogger Blog:-

The XML Sitemap file is like a directory of all web pages that exist on your website or blog. Search engines like Google and Bing can use these sitemap files to discover pages on your site that the search bots may have otherwise missed during regular crawling.
The Problem with Blogger Sitemap Files
An complete sitemap file should mention all pages of a site but that’s not the case if your blog is hosted on Blogger.
The default XML sitemap file of any Blogger blog will have only the 26 most recent blog posts – see example. That’s a limitation because some of your older blog pages, that are missing in the default XML sitemap file, may never get indexed in search engines. There’s however a simple solution to fix this problem.
Generate a Complete Sitemap for your Blogger Blog
This section is valid for both regular Blogger blogs (that use a .blogspot.com address) and also the self-hosted Blogger blogs that use a custom domain (like postsecret.com).
Here’s what you need to do to expose your blog’s complete site structure to search engines with the help of an XML sitemap.
Open the Sitemap Generator and type the full address of your blogspot blog (or your self-hosted Blogger blog).
Click the Create Sitemap button and this tool will instantly generate the necessary text for your sitemap. Copy the entire generated text to your clipboard (see screenshot below).
Next go to your Blogger dashboard and under Settings – > Search Preferences, the enable Custom robots.txt option (available in the Crawling and Indexing section). Paste the clipboard text here and save your changes.
And we are done. Search engines will automatically discover your XML sitemap files via the robots.txt file and you don’t have to ping them manually.