Research Assistantships are typically arranged independently by students and faculty members. This is where networking pays off: let all your professors know you are interested, and ask if any have grant funds to pay you, or if they know of any researchers who do. When your advisor is writing a grant application, suggest that the funding for a graduate student be written into the budget.
Research assistants can be paid two ways: if the hours are irregular, they should be paid on the temporary payroll, which is administered by Joan Smeltzer in the Department's main office. The student turns in weekly time cards, signed by the supervisor, and is paid weekly.
If the job is more predictable, the student can be appointed a research assistant on an appointment processed by the Graduate Office, and paid monthly. There are no time cards to be filled out. If the student is also a Teaching Fellow, the Harvard Payroll Office combines the salary for both jobs into one paycheck, and the student needs to pay careful attention to what the salary should be, since neither the Payroll Office or the Graduate Office are infallible.
Research assistant salaries can and do vary, within a range of about $9-20/hour or more. There is no FAS-set salary, as there is for Teaching Fellows; the rate is negotiated between the student and supervisor and is dependent on the degree of skill involved and the constraints of the research grant.