There are a few methods of doing it, either using the pressure of the heatsink to spread the compound, or using a straight edge like a razor blade or a piece of card to spread it. It is isn't crucial to get compound right to the edge of the head spreader, and could prove messy if it leaks over the edge. The most important thing is getting an airbubble-free contact in the center region of the heat spreader, directly above the cpu itself.