C# - insert macro into word document using C#

Question

I am trying to insert a macro into eight Word documents using a C# program.  I can open a word document and insert various things  (particularly mailmerge, as that seem to be pretty prevalent on the net), but I cannot find any clues, that I understand, to point me in the right direction. I have the required macro in a .bas file and it just need slotting into my eight documents so that if I need to modify it I only have to do it once.  Can anyone help please.