Ok, this comes from a person who claims he can speak Russian ... little do they know, but ...

The best way to speak fake Russian is to know a few usefull Russian words. I recommend "durak", "nyet", "chevo?" and of course "ty porosyenok". Mix and match these words for added bonus.

Be very articulate. Move your hands around and sometimes attempt to hit the person you're talking to on the shoulder. That is considered a sign of affection. Honest.

Practice your best fake-Russian accent. Watch early Arnold Schwarzenegger movies where he tries his best at Russian. Those movies are truly priceless.

Be creative. If the person you're talking to actually can speak Russian, then do your best to persuade them you're from a different region of Russia, and that you're the only one who actually speaks Russian correctly. Use phrases like "Grrr", "Nyu Nyu" and fake losing interest in the conversation.

For truly excellent results, don't excessively yell at the person you're talking to. Some yelling is permitted, but don't overdo it. You'd be better off that way.