When applying Thermal Paste I prefer to add a small amount around the surface of the CPU. Then using a plastic card I carefully spread the compound to cover the whole CPU. Basically as long as you get the whole CPU covered you're going to be alright. Just be careful not to use too much. Going overboard on the amount of compound used can actually hinder it's ability to transfer heat off the CPU to the heat-sink.
Really, as long as you've got a thin layer that covers the CPU and you can't see through at any point, you should be fine. Just take your time.