Same way you always did
Start off in the Sol system , with a standard setup. The population etc of the starting human race is what is going to be used for the difficulty to set that to what you want for your race.
Then either go to a Earth on the system screen and select create a new race OR and more likely use the create new system command until you get a system you like, you can create a colony from the human starting race on a planet and then terraform to suit, once you have a planet you like use the create new race command , obviously make sure you select the player race option then procede as normal.
You can now delete the starting human race and any systems you want to get rid of, you could also create an NPR human race on Earth if you want