creating custom define cube

hi

i have a situation here. i need to create a ods . i need to pull the data from r/3. where all the tables are not standard tables . now i what r the steps to create the ods . and pull the data from r/3 tables . can u plz explain . this is new to me . i was doing all the sta ndard content till now . and i now have something to do were it not standard i now flat extraction but i dont know how to from R/3. can u help . thanks in advance