Just put a small amount then actually spread it to a thin layer. You want to make sure it all has some on it. Make sure it is always a thin layer though, you don't want it being too much and squeezing out the sides. Then you just put the heatsink on, you don't wait for it to dry.
Even if you don't do this, it still shouldn't matter that much. What is the temp rising to? Is it overheating or something?