First off, create your simBrief flight plan, which it sounds like you know how to do.
Once you are happy with the plan, press the ‘Generate Flight’ button.
When simBrief has finished generating the flight, scroll all the way to the bottom, to where it lists all of the file types you can export to. You are looking for the ‘FS2020’ option, which will name a file with a “.pln” extension. Click on ‘Download’ and it will put the file, very quickly, into your usual Windows download folder.
Go to the MSFS World Map screen and from the options along the bottom of the screen, select ‘more’, then select ‘load/save’. (i’m not at my sim pc atm, so these may not be spot on, but it should be obvious which options i’m talking about).
Navigate to your download folder and select the file that you’ve just downloaded and press the ‘OK’ button. That should load the flight into the world map, in enough detail for the MSFS ATC to provide guidance. You should be able to move your starting point to a stand or parking spot.
Press the ‘Fly’ button and once on the tarmac, you can go through the usual process of loading the currently active simBrief flight plan into the MCDU and into the EFB.
Make sure you have the ATC window open and interact with that as appropriate, throughout the flight.
Happy flying.