There's not a whole lot to it, really.
Go to
the Missions section of Shadowrun4.comDownload the adventures from the downloads section.
Use archetypes or generate standard 400 BP characters.
There's additional information available from the FAQ in the Missions section and the notes to players & GMs.
If you have specific questions, you can ask them here, drop me a PM or send an e-mail to missions@shadowrun4.com