Starting in "The Old Gods" Bookmark (Requries Old Gods DLC) Select the Character Rurik of Holmgandr he should be marked as character of special interest. He currently is not of russian culture making this impossible. (for now)
Now start the game in ironman otherwise you can't get the acivement.

Now that your in the game take the ambiton "Become The King Of Rus" which is actually all you need for this acivement. So now using your Subjugation or conquest CB declare wars on the surrounding Counts and Dukes. Leave the Duchy of Tver alone untill you have conquered all nations smaller than they. And once you hold enough land BAMM half of this is done already now for the next part.

Now if you did not already notice your not Russian at the start of the game BUT you start in a bunch of russian provinces so go into your intruige menu and find the choice "Convert To Local Culture (Russain)" (Edit: apparently you need Rajas of Indai to convert so just educate your child with russians until you get one of them of that culture.) If you have the prestige which you definatly should by now take the descion and your done
There you go all done now if you want to make this into a longterm game and go for other acivements like "New Ways for Old Gods" or "Not a Tribe" i suggest you just keep playing and use you power to do what ever you want. (Maybe even Emperor of Russia one day) :)
Heres what i look like after 10 years.

I really do thank you for reading this Guide its my first one so forgive me if it was awful but if you found it usefull please rate up it really helps and stay tuned for more guides

Bye!