OMG, worst design ever... I put mine on last week and it was a pain in the ass...
The instructions that come with it are correct, until you get to put it on the tank.
I took the fitted nuts and laid them in the tank and lined them up where they needed to be, while having the DP top part off the stock bottom part, as the 4 bolts that go into the tank itself cant be accessed with the DP piece on the stock lower half. So, get those 6 or so nuts laid out around the recessed area on the tank, lay the stock lower piece over it, realign the nuts as needed, and tighten the 4 main bolts to the tank, SLOWLY. The DP nuts will walk on you if you're not super careful.
Then, put the DP top part over the stock lower, and very carefully start to thread the bolts into them.
It's annoying, but you can do it. Just DON'T drop one of the nuts in the tank, it's a bitch to get it out. Ask me how I know :^/