Fandak: Your summary is almost true, although it is actually done the other way around. That is, your chosen avionics installer prepares a design change document (which will usually be based on the manufacturer's installation materials) and submits it, along with the draft AFM supplement, to EASA for approval. When it is reviewed and approved, then they can go ahead and do the work. It would be an expensive mistake to first do the work, and then try to get it approved, in case they have got it wrong somewhere.