display recent network post in homepage?

Good evening,
this is the first time I write on your forum. I need help setting up wordpress multisite for a news site. I was able to activate without problems WPMU, the thing that I can not understand is how make sure that the posts on the sub-blog, appear on the homepage.

example:
I have 3 sub-sites
-site1.example.net
-site2.example.net
-site3.example.net

I need to show the news (the latest 20 news) published on these sites on the home page of the blog (example.net).
How can I do?
How do i pick items around the network (for example, by the slider)?

thank you