Show only som few lines in blogpost

… and read more to see the whole article. How can i do that?

If I understand you correctly - this is built in to RW blog. Put small amount in ‘summary’ area and the rest in the ‘body’ area.

In ‘inspector’ > ‘general’ go to Permalink settings and change ‘read more’ setting if required, to other text

So easy! I’m not very familiar with RW yet. Problem solved!