General SQL question

Hi, all. This question is not directly related to Informix, more a generic one. Still, I always had received very good help in this ng. Thing is, I am responsible for database design in one DWH solution. It is quite complicated, about 100 tables and growing. Recently analysts decided that, in regards of latest business analysis requirements, DWH has to contain reference table between two tables, as they are in many-to-many relationship. Now, question is - how to fill keys in this reference table? Data load process at first fills data in respective tables, then updates foreign keys between tables and theh time would be to fill reference table, which contains only two fields, foreign keys to two other tables. I have no idea, how to write procedure, which would fill and update reference table, can you suggest some exapmle SQL script? Thanks!