firstly secure the processor in the mainboard processor socket.
then apply the thermal paste about 2x the length of a grain of rice on center part of the processor.
then secure with the heatsink.
do not apply too much as it may slipped onto the mainboard.
A 'pea' size amount of thermal paste in middle (on top) of the processor should be ok.
Most Intel® fans comes already with the thermal paste already applied on the heatsink, this causes less trouble for people.