I want to convert them to a wordpress blog. Please suggest how to do it without loosing traffic
You want to make sure that every old URL (ie blogger) 301/permanently redirects to new URL.
Another great but more recent step by step tutorial can be found on http://www.wpbeginner.com/wp-tutorials/how-to-switch-from-blogger-to-wordpress-without-losing-google-rankings/
You can use the WordPress default importer tool to import all the posts from blogger.com account. After that you can go with Blogger to WordPress plugin which will be useful in one to one redirection of posts.
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