That first link is to the double fisherman's knot that you make your loop with, then click on the further link to the prussic knot. Make sure your main rope is a larger diameter than your prussic loop material. If you use high quality climbing rated rope, it will cost you about $30 for a good set up. You can make one with decent cheap rope for half that. One fall with a minor injury will cost hundreds, if you are lucky. One out of three tree stand hunters falls eventually.
As Hans and Frans of SNL used to say, "Hear me now and think about it later."