Make sure to pick a large, ripe watermelon. To check ripeness, knock on the watermelon. If the knock sounds like a dull thud, it is underripe. If the knock makes an almost hollow-sounding noise, the watermelon should be ripe. Make sure that the watermelon has a round, regular size and feels heavy. When fruit feels heavy for its size, it means that it has a lot of water inside and is ripe. Wash the watermelon, then lay it on a cutting board. With a large knife, Peel a Watermelon by first cutting off the very top and bottom, then placing the watermelon upright and slicing downward to remove the rind.  Make sure to keep your fingers clear of the area of the watermelon you are cutting. Also use a sharp knife so that you don't run the risk of having to use excessive force and accidentally nicking yourself with the knife. After cutting off the rind, slice off any white strips on the watermelon until you’re left with only the red fruit. After you have removed the rind, chop the red fruit into one-inch (2.5 centimeter) pieces. These pieces don’t have to be precise since you’ll be cooking them down, they just have to be relatively small chunks. Place the watermelon chunks and the juices in a large pot and turn the heat to medium. You will cook the watermelon down so it liquefies and can be turned into wine. As the watermelon heats, it should begin breaking down. Help to speed up the process by mashing the fruit with a large spoon and stirring the watermelon frequently. Stop when most of the fruit has broken down, about a half hour, and remove the pot from heat. Carefully pour out 14 cups (3.5 liters) of the watermelon juice through a fine mesh strainer to catch any of the watermelon seeds or large chunks of the fruit. If you have leftover juice after you pour out the 14 cups (3.5 liters), you can reserve it to drink cooled or to use in cocktails. Store the leftover juice in a sealed container in the refrigerator for up to three days.

TL;DR in Indonesian:
Pilih semangka yang tepat. Buang kulit semangka. Potong semangka menjadi kubus-kubus kecil. Masukkan semangka ke dalam panci besar dan masak buah. Aduk dan hancurkan semangka hingga mencair. Saring sari buah.