For starters, you forgot to mention what type of cuplex water block you have so people won't be able to give you a lot of details. But basically you need to take apart your existing cuplex in order to attach the new plexiglass top from the conversion kit onto it.
Make sure you keep the screws from the original block because you might need these to attach the new top. Look at how the cuplex base is attached to the original top when you disassemble it, that will help you to put the new one on it. And don't forget to put the O-ring in between the base and the new top, otherwise your setup will leak coolant into your system.
Once you have the new top on the cuplex base then the rest should be easy to put together.