Archives in the former Soviet Union have a hierarchical 3-part identification of specific documents. Fond might have hundreds or thousands of "opis". Each opis might have hundreds or thousands of "delo" (the actual documents). These documents in turn could be anywhere from a handful to thousands of pages. When you request documents from the archives, you need to be precise in what you ask, if you expect to get a quick and exact turnaround. This request is almost an equivalent of writing a letter to your city's central library and asking them "I understand that you have books that start with a letter M and someone once borrowed one of these books and translated it into another language. How can I take a look at these books?"
If you found some lists online with the census information, most likely they will contain a reference to the actual archival documents and might look like "NHAB 2151-444-13". They should also have a name of the archive, most often an acronym - NHAB (National Historical Archive of Belarus), or NHAB Grodno (National Historical Archive of Belarus at Grodno), or LVIA (Lithuanian State Historical Archive), etc. Think of that name as a city, fond - street, opis - building number, delo - apartment or office.
From what I understand, Belarus archives will not send you the entire census/vital records book (unless you are onsite in their reading room), only scans of the specific pages, records, or families in the document. You will need to specify who you are looking for in the document.
If you do not have exact ID of the document, try to be exact with what you are looking for and skip any information that is not going to be relevant to the archivist - it's not relevant to them what the relationships are, where these people lived after they left the area, etc. You will need to know their names as they used them in the old country, if the names changed after they came to the US, that's not going too be helpful to the archivist. Specify that you looking for a person named X Y, who lived in town Z in years 18## to 18## and they were of Jewish faith. You are looking for census and metrical (vital) records that might contain information about this person. List other family members if necessary. Having precise dates or at least a range of dates will help them look for the specifics or identify which records might be missing, that would have information about the person.
I have not dealt with Belarus archives myself, but from what I understand, they are easy to work with.
P.S. as far as I know, RAGAS is Russian-American Genealogical Archive Service.