The Moscow embassy website has some info on this and it is very hard to get good step by step info on what to do. We are trying to do the same thing and just found we were doing some things wrong last night.
Anyway, from what I read you have to fill out the DS-156 online and print it, than that goes to your MIL who must take it to a local pony express office and mail it along with her passport, two passport photos, a payment of $ 131.00 US but in Roubles, any supporting documents etc to the embassy. Pony express will let her know when the interview is at the time and it will only be a week or two away then she must go to the Embassy for an interview. I don't think the interview can be done in S.P. I think Moscow is the place.
You can check it out on the Moscow embassy site and see if there is more info or if what I am saying is correct.