After I created a blog on an instance, I saw that all my articles will be shown in full length on the main page of my blog. But if I go to the main side of the instance, all blog articles of all people are shortened and there is a “read more” button.
Is it possible to create this “read more” button for my sub blog? Or is there even a trick to shorten the articles in the overview with custom CSS?
The Reader page will automatically shorten posts, but your blog won’t. You can manually set where that happens with a little shortcode, which both your blog and the Reader page will respect:
Cool, this was pretty easy. Thanks a lot. 
FYI - If someone tries to do an index:
I figured out this doesn’t work if you want to create an index only with head lines and no previews. Putting the line <!-- more --> right after the main head line doesn’t make any changes. It must be placed after a normal text.