I've been doing this a lot so I'll help you out. When you click new game, there should be an option on that big screen called "Sol Start" or "SM Race start - Advanced players only". Click the SM Race start. Some of the other options, specifically those relating to your race, become irrelevant but some don't. Just set everything up the way you want it to be safe.
Now when the game is generated, you will start without a system or a race. Before you can do anything, you will have to select your default race in the 'Game' menu. Set it to your only option, SM Race. Also set spacemaster on. Next, press F9 to go to the system screen. One of the buttons near the bottom right should be 'Create System'. Click it. A new system should be generated. This should get you started, just remember to delete all the unused systems you generated.
Eventually, when you have the perfect planet for your race, press the 'Create Empire' button. You then get to put in all your race options again. Have fun!
