Graphicly using the pedigree chart, since you have both birth parents, you place them in their places and just don't trace back to the unwanted lines; just nip it in the bud from that point. Otherwise, if you don't want the birth parents listed at all, a connection from the grandparents with a note of "adoption" is all that is needed. The full story and history can be added in the written version to explain the connections.