one set of instructins sez:
 
Once the lever is lifted, apply an even layer of paste on the surface of CPU
 
then later
 
before installing the heatsink, please add an even layer of paste to the CPU.
 
Do I add two layers of paste or one?
 
I only add paste to the smooth side of the CPU... definitely not the side going into the socket.
 

one, to the top of the CPU(smooth side)

Yes and no, you can apply thermal paste to either the processor (smooth side) or the heat sink, but not both.

Lock the CPU socket arm down first. Then add the paste. I prefer to add the paste to the top of the CPU. The base of some HSF's are larger that the CPU heat spreader. Applying the paste to the HSF may result in excess paste getting squeezed out to go who knows where. That seems less likely if you apply it to the top of the CPU.

Some heatsinks come with paste preapplied.
If that is not the case it's better to apply it to the CPU IHS.