The blog exposes a feed link (it only works after there is at least one post in the blog), right click the feed link and choose copy link location. Then paste that as the feed url in feed manager, put the url for the blog page as the web site url and whatever you like for the title.

If you click the edit links for the feed manager on the demo site you will see what I entered there.

The feed link in the blog is right below the calendar

right click it and get the feed url

I just told you where it is. You only need to add it once to the feed manager not for each post, I already added it so you don't need to add it on the demo site you just need to understand what I have already told you.