Thanks to their air force of flying pigs and their sealion navy, I suppose?
As I said, you're making this a DBWI because there's no way you can put together a plausible TL out of it.
Flying pigs are not needed to conquer the place, given certain divergences from OTL. A Russia which built rails east earlier than OTL _might_ be able to conquer China during the 19th century, if we can somehow keep the UK or a UK-equivalent out of the game. Or a more successfully modernizing Russia gone fascist might conquer Russia in the first half of the 20th century. Much bigger industrial base and manpower pool than Japan. Holding onto it until 2009 is another kettle of
garum.
You would need Draka-esque savagery and social engineering to smash Chinese society completely enough to pacify the place long-term, and what does Successfully Modernizing Russia need impoverished Chinese serfs for? They already have a massive workforce, and a better educated one to boot: it's not only extremely difficult, it's not of any real _use_, either.
Slightly more plausibly, if the Mongols convert to Russian Orthodoxy, you might be able to pull of a China ruled from somewhere within European Russia by the 1400's...of course, at best it's going to be Chinese Russia within a couple centuries.

(Scottish kings of England will always gravitate to London).
I can see a "Russian China" in the sense of:
1. Russians move eastward earlier, conquer Manchuria before it fills up with Chinese settlers. Later immigration from a China which has as a bad a century as OTL means that by 2009 Manchuria has a Chinese plurality, and the area is sometimes referred to as "Russian China", which the Russians _hate_, since it reinforces the propaganda of the nasty leftist Chinese regime which claims Manchuria as stolen territory.
2. Russians move east a _lot_ earlier (gunpowder arrives in Eastern Europe earlier? No Mongol Empire?) in a TL where Russia never becomes a unitary empire, and Siberian Russians play a similar role in China to the Manchus OTL. The Russians maintain their Orthodox religion, but otherwise become quite sinicized.
3. Something like "decades of darkness", in which a big chunk of a divided China is part of a Russian-led federation as protection from being swallowed by a thoroughly nasty regime in other parts of China.
4. A much closer Soviet-Red Chinese alliance founded by Party leaders with very different personalities than Stalin and Mao. It's actually a partnership, but all the Capitalist Running Dogs assume it's Russian-run.
5. And of course, the genocide option, in the fine tradition of Jack London's "The Unparalelled Invasion." Chinese-Soviet war (different Chinese and Soviets than of OTL) goes very badly when the Chinese turn out to have a few missiles which (a) the Soviets didn't know to get in their first strike and (b) can reach Moscow and Leningrad. The vengeful Soviet military, with no orders coming from a mostly dead Party Leadership, turn to a strategy of "make the rubble bounce." Later, after the radiation dies down a bit, the Soviets move in to take over the place from the 20-30 million surviving Chinese: for one thing, South China is warmer (it's a nuclear chill if not a full-blown winter), and since the rest of the world now thinks of them as Crazed Genocidal Bastards, they should at least get some territory out of it. Better to be hung for a sheep than a lamb and all that.
Of course, none of these gives us a united (and radiation-free) China under the rule of Moscow (or St. Petersburg).
Bruce