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First of all you Should verify your Website in Webmaster Tool .
- Go to Home Page/ Dashboard of Web master Tool. Then Select your Blog that you need to submit your Sitemap.
- Select Sitemap from Site Configuration .
- Select "Submit a Sitemap".
- Now , Automatically your blog name will be show there ,Now jut suffix rss.xml and Submit it.
- Done it !. Look how Simple is.!It take some minutes/hours to Index .No need of External web tool..