I'm rather rusty with this process myself, however I'm about to add orcs to my WH 40k game so I'll be reminding myself how it's done.
Edit: I think parts of the tutorial on the wiki might be slightly vague so I'll edit this post with a step by step process for creating an alien race, and also adding another empire in the sol system too if you wish.
First I generated a new game changing only 2 options from default, select "Create spacemaster empire" , and change "number of computer controlled NPR's at game start" to 0. You'll also want to set the NPR generation chance to 0 so that NPRs don't get generated while you're looking for a suitable planet.
When you select the game it loads up as "game name" January 1st 2025 00:00.
Next under "game" click "select default race", you'll see there's only one option" SM Race" select that.
You need to turn on SM mode now in order to have the next option you need visible on the system page, so select "Spacemaster on" under "spacemaster" on the main menu.
Next turn on the system information screen (F9).
Click the "create system button" in the current version (7.0) you'll get a list of systems, theres around 1300 in the initial list, just select one and hit "select", I choose Rigel because I like trinarys :p
After a second the system should be displayed, you'll see information about the stars and then tabs for displaying the bodies orbiting each one.
Now you need to select a planet for your new races homeworld, in order to be suitable the planet must have a breathable atmosphere with Oxygen atm from 0.07 to 0.40 and no dangerous gases like methane, sulfur dioxide, fluorine and others. Note that you can swap oxygen for methane if you want to create a methane breathing race.
It is possible to use SM mode to modify the atmosphere, you need to add a colony there first then use SM mode on the atmosphere tab of the production screen to modify it to however you need.
When you have a suitable planet select it and click "create empire"
There's a lot of options you can change here, mostly the ,same as those available when you first generate the map, however additionally you can also set the race as neutral or as a computer controlled NPR race.
When you're satisfied with the changes click "create Empire" and the race will be generated with the selected planet set as it's ideal environment.
Now if you wanted to also create a race of humans on earth you can do that, however don't use the "Create system" button for sol as it will generate a random system, also it's slightly bugged and will give a binary. Instead on the main menu bar under spacemaster select "Create sol system", you'll need to close and reload the system information window in order for sol to show up, then just select earth and create an empire there.
Now if you want to change the NPR generation chance, just go back to the game info screen, remember to save before selecting the game again, and finally if you want to add a random system with a computer controlled NPR there is an option under "Spacemaster".
Good luck.
