This step is all about selecting the best side of your disc to be top and the less desirable side to be bottom.
Inspect your oak blank that you have transformed into a disc and based on the wood grain, knots and any imperfections from your cutting and sculpting, select which side will be best suited to be highly visible as the top and which side will be best suited to be on the bottom.
For me, I had some tear-out where small pieces of wood were torn out of one side of my disc during sculpting on the lathe. I decided to put the torn out side as the bottom and the other side as the top.
Using your pencil, write "bottom" on the bottom side so you do not get confused in future steps!