Alright so I will list the steps on how to do the dupe..

1. You have exactly 1 gp less than the max value in the money pouch (You have 2,147,483,646 gp in your money pouch)
2. You add 2 coins to it, so that it exceeds the maximum value of the money pouch
3. One coin goes into the money pouch making it maxed, but the other coin that is in your inventory turns into another maximum cash stack.

I explained that the best I could here's the link of the tutorial I followed..
http://www.rune-server.org/runescape...supported.html