It's easy.
http://www.youtube.com/watch?v=o7rPqCvCt0g
I wouldn't recommend spreading the stuff though. Put the cooler on flat and the paste will spread out itself pretty good. The biggest thing, don't use too much. Little more than a grain or two of rice is usually about perfect. After that, put the cooler on there slowly and apply pressure with the clamping system. Usually the pressure alone will spread the paste extremely well and work great.
Use your noodle here! Try it a couple times before bolting it down and going with it. Try a certain amount and make sure it works by pulling the cooler off. If it has spead over the CPU (at least 90-95%) and the heatsink; then you're pretty good to go.
What you need?
Thermal paste of course
Isopropyl Alcohol 70% or higher
A lint-free rage
Clean the CPU and bottom of the heatsink with each removal. Don't just some some on and close it up without cleaning it. That can be the worst thing to do! So clean it up; try a little bit and mount it. Then pull it back off and make sure it spread good and evenly. Then clean it back up and try again until you get the amount you need and then put it together.
Hope this helps!