Hello mate.
To save you time, then keep the CPU in the MOBO. Just make sure there is no power plugged in.
Clean the thermal paste off both on the CPU and the water block. You don't want to get your fingers on the water block or the CPU. Oils from the human skin can decrease the cooling performance.
To clean the CPU and the water block you don't need to go shopping for some expensive CPU removal liquid or cloth. You just need some rubbing alcohol with over 70% and some ear cleaners, or ear pins whatever the name is. Don't use paper or cloth that has fibers in them, fibers will stick to the surface of the water block or the CPU and it will decrease performance. The reason being of using those, is that they soak liquids up easily and it makes it easy for you not to squirt alcohol all over you system, also they don't have any fibers in them. Dip the cleaners in a little of the alcohol and wipe it over the CPU or water block. When the thermal paste have begun to losen up, then take some dry ones and wipe until they're dirty. Again use some alcohol dipped ones and make more losen up. Repeat doing so until both are completely free.
Let both dry for about 5 minutes, so you're sure no alcohol is left on them.
When you apply the thermal paste, then there are a lot of opinions of how to do so. I found the easiest and most reliable way to just put a pea-sized drop in the middle. You don't have to spread it or anything.
When you tighten the water block, then do it balanced. THIS IS VERY IMPORTANT. Tighten one screw a little, then the opposite, then the screw next to it, then the opposite and keep on doing so until it's completely tightened.
If you do this properly, then the thermal paste will be almost 100% even spread out on your CPU and you'll get the best possible performance.
I hope this helped and good luck mate!