I actually did a Watermelon Kolsch that came out quite nice.
I only used a small watermelon, one of the ones that are about basketball sized. I probably only had 4, maybe 5 pounds of watermelon and it worked well (sorry, didn't measure how much actual juice there was).
I juiced mine in a food processor, poured everything through a steel strainer, then strained it again through the fine screen in my big funnel straight into the secondary. I did not pasteurize, figuring the alcohol would kill anything nasty off. I was very careful when I cut up the melon into chunks, sanitizing the board, knife, processor parts and such. Other than that, I didn't worry too much.
Like DubbelDach mentioned, it kicked off a second active fermentation for a day or so and I ended up with a big layer of trub in the carboy; just like the picture posted.
I really wanted a Watermelon nose and flavor so I also added one dram of Watermelon Extract from LorannOils.com (https://www.lorannoils.com/c-6-super-strength-flavors-candy-oils.aspx). I'm a big fan of their products and 1 dram (aka 1 teaspoon) of their extract is usually enough for a 5 gallon batch, depending on how strong you want the flavor.
I actually racked to a tertiary for 3 days because of the wicked yeast cake in the secondary. It worked out well, but probably wasn't necessary. 10 days in primary, 10 in the secondary and 3 in the tertiary.
The amount of Watermelon and juicing vs extract or both is up to you and what you're going for. I knew I wanted everyone to say "hey, that's watermelon, but it's beer!", so I went for both. The one thing to keep in mind is that if you over do it, you can't go back. Under do it and you can add some flavor via extract at bottling.
We're enjoying this batch now and it's exactly how I wanted it... definite watermelon nose, some flavor, yet all beer. Pretty nifty if I do say so myself. My friends that have tried it think it's great and want more, so I may have to do another batch before summer's over.
Good luck! Don't worry too much and have fun. It'll still be beer!