Slice the watermelon into triangles. Cut away most of the red flesh while leaving some lighter pink flesh behind. Trim off the green skin and cut the rinds into small chunks. Transfer to a large bowl.
Toss the watermelon rinds with salt and let them sit for 60 minutes. About halfway in between, toss them again to distribute the salt. After 60 minutes, rinse the rinds under cold tap water, then use your hands to squeeze them to remove excess water. Transfer to a large, clean bowl.
Now, let's make the kimchi paste. Place the watermelon flesh in a food processor and blend into a puree. Add the garlic, ginger, onions, and fish sauce and blend until smooth.
Add the kimchi paste, gochugaru, and scallions to the bowl of watermelon rinds and mix until well combined. At this point, you can eat the kimchi immediately, but I recommend letting it ferment at room temperature for 1-3 days.
To ferment, pack the kimchi tightly into a glass container, loosely cover it with a lid, and ferment at room temperature for 1-3 days. I like to check the flavor of the kimchi every 24 hours. When you are happy with the flavor, transfer it to the fridge where it will keep well for up to one month.