Where does the disk go?

Part of what my supervisor is doing for her project involves growing bacteria then challenging them by putting little antibiotic disks on the plates and seeing if the bacteria die or not. She can't make it in today, so she asked me yesterday if I could put the disks on the plates for her.

The procedure was fairly simple and wouldn't take long, so I agreed. I came in this morning all bright and early, and suddenly realised I had no idea where on the plates I was supposed to put the disks.

It the middle? At the edge? Where the bacteria are growing?

It's my fault for acting so competent in my first week. It made her form the impression that I know what I'm doing. :(