I did mine back before I started keeping better notes but as I recall it was very simple. Keep in mind I’ve never had 21a but read alot about it in my research.
6 Gallons
4lbs 2-row
4lbs Wheat
1lb American Lt Munich
Not sure on the hops but it would have been a low bittering charge only. 15 ibu or so.
After the vigorous fermentation was over I scrubbed the outside of the watermelon with dish soap and a sponge. I used steel wool to get the dirty spots off. I then rinsed and sprayed the outside of the water melon w/ sanitizer and sanitized the knife, cutting board, bowl, and colander. I squeezed the water melon through the colander to extract the juices and discarded the fiber/pulp. I got a little less than .75 gal of juice into secondary and racked the beer on top. Fermentation took off again. It turned out real good and I am doing another in a few weeks.