No, I buy them whole from sbherbalremedies.com. You can grind them up with a coffee bean grinder. Even better would be something more heavy duty like an Ultimate Chopper (really strong blades, 750 watts). I grind until it's somewhere between granules and a white powder.
I put about 2 tbsp into a glass carafe (left over from my French press coffee days. funny how all the accessories from my coffee habit are employed in my cleansing protocols). Holds about 24 oz of water. Pour boiling water in, just like making tea, and cover. The Cayce recipe says "let it cool." http://curezone.com/cleanse/kidney/Watermelon-Seed-Tea.asp
I've let it sit anywhere from 1/2 hour to overnight. But unlike nettle leaf, for e.g., it's not one of those things that gets stronger and stronger as it sits. Seems to me after 1 or 2 hours it's completely steeped (done). So pour the water, then go do something, maybe run an errand. Then strain it--cheese cloth, old white pillow case--and drink. Warm, cool, no diff for me.
I still have some seeds left, and I'll use them eventually, but I think the chianca piedra is stronger. Although I do eat Watermelon just about every day. All the supermarkets have a little section of halved or quartered watermelons.
I've never seen any stones in my urine. I don't think it's that common. When I look at illustrations of the kidney and look at the renal pelvis, I think of kidney stones as something to be dissolved, slowly, over time.