So, you've bought Starfield Shattered Space, and now you want to know how to start it? Look no further!
To start the Shattered Space expansion, you'll need to complete the first mission of Starfield called One Small Step. This is the fairly lengthy tutorial mission that sends you out on a quest and ends with you talking to Sarah in the library.
Once you've done this, you'll need to grav-jump to an orbit that isn't being used by a mission or encounter, which will prompt a distress call and unveil a large star station called The Oracle. This is where the Shattered Space DLC begins.
So, it's that simple! All you have to do is complete the first mission of Starfield, and then you'll be able to access Shattered Space. The big thing to be aware of though is that Bethesda recommends you're Level 35 before checking out this expansion, so anyone who's under that level may find it a lot tougher than other players.