The graphics card block is a little trickier to fit, so I recommend a thorough read of the instructions before doing anything.
Unscrew any bolts holding on your current GPU cooling block and put them away in a plastic bag so you don't lose them. Once all screws, bolts and retaining mechanisms are off, you can gently try pulling the block off the card.
If it doesn't come off easily, then you are unlucky like me and the thermal gunk applied by the manufacturer is either an epoxy or has just set rock hard so that your block is stuck down.
If this is the case, do some looking online and find an article on removing GPU blocks, they usually involve freezing or heating the block to make the gunk more brittle or pliable respectively.
With the block off, gently scrape the remaining gunk off the core with your thumbnail or a soft plastic edge. And for the love of god, don't use anything too sharp or too hard! You will damage your GPU block.
With the gunk adequately cleaned off, you can apply new gunk to the core. A nice even coating should do the trick.
Following the instructions, placing the retaining bracket on the back and fitting the screws through the holes, flip it over and place the new GPU block on top of the core. I recommend letting it face towards the top edge of the card so that fitting the water cooling tubes is easier.
Loosen the collars off and position them over the screws coming through from the back side. Then tighten up all the screws with equal tightness; the block does not need to be super tight on the card, as the retaining bracket behind is only aluminium and will bend.
At this point it is worth mentioning that if your original cooler block cooled the RAM blocks, then you will want to buy some cheap copper or aluminium RAMsinks to stick on them as a replacement.
Also a slight concern on my 9600GT card is that there are some surface mounted components right behind the core and when the retaining bracket is fitted, the rubber rests on top of these components. For this reason, it's critical you do not over tighten the bracket or you risk damaging the card.
When you have successfully mounted the block, you can pick the correct size hose adapters and fit them to the ends of the GPU block hoses. Make sure you tighten then down real good so that the o-rings sit beneath the thread.