How do you get Hoenn starters in Pokemon SoulSilver?

Answer 1

You can only get one Hoenn starter per game without trading or cheating. After defeating Red, Steven of the Hoenn region will be in Silph Co.'s building. He will give you three colored stones (each color corresponding to the Pokemon type. I.e., blue stone goes to Mudkip, etc.). Choose the one you want to get that Hoenn starter.
First talk to the copycat girl in saffron city and then go to vermillion city and get her plush doll back but while your walking out of the house steven will talk to you and then beat red and go to silph co. and talk to steven torchic= red mudkip=blue treeko=green (i recomend mudkip)

Answer 2

Ok, so after you beat Red, and the game goes to the credits, go to your saved file. After that, you should be in your room. Fly to the Kanto region and go to silph co in Saffron city. Steven should be at the counter to the right of the front entrance. He will say "which one do you prefer? Red, blue or green?". If you answer green, you will receive a Treecko. If you choose red, You will receive a Torchic. And of you choose blue, you will get a Mudkip. I think they are all at level 5 when you get them.

Hope I could help~!