I am guessing this is for the CPU? I just did something similar earlier this morning with my graphics card and heat sink on the GPU. It is, of course, a LOT smaller in size (I think) - in the video I watched it was generally recommended you put the paste about the size of a grain of rice, however you can add a little more. It will 'squish' out and you don't want to go crazy with it- I don't think.
The most challenging part was removing the old thermal paste- not from the processor (that came off easy- and I just left the old paste around the edges there)- but getting it off the copper bottom of the heat sink was QUITE the task, especially since I lacked 1) alcohol of a grade to remove it without residue, 2) my wife apparently has nailpolish but no nailpolish remover... !!! which is apparently a good substitute and leaves no residue. So instead I used the cleaning spray I use on my 3D glasses and TV/monitor green (that also says it will leave no residue)- but it isn't that great at removing it. I used some plastic scraping tools from my electronic repair kit, but one big trick I learned that made all the difference at the very end?
Use a hair dryer on it. I turned it up to max heat, and put it right over the paste on the heat sink- it heated the heatsink up too, but exactly like the fellow said, the paste came off SO much easier! So- I highly recommend using that trick when working with removing paste, even if you have the proper liquids to help. I also used a paper towel and, for final cleaning, the dusting/cleaning towel that came with my screen cleaning kit and glasses. With the copper clear and clean I was ready- the fan blades hand dusted and the heat sink dust sucked off with my vacuum cleaner- and I put it all back together.
So- good luck! The key is just making sure you get it clean from the old stuff so you get a good, clean contact with fresh paste. Apparently, also, replacing old paste with new can help speed things up if heat issues seem to be arising.