i don't know if it's applicable to laptop, but on my desktop, i would usually put approximately in a size of rice grain,
so when u tighten the heatsink, the thermal paste will be evenly distributed to the cpu surface

well, that's the easiest way i supposed, let it evenly distributed naturally,
some people prefer to spread the paste by their own, so need a good even surface such as credit card, quite tedious i supposed