There is a specific tool for this:
You could do the same with a piece of aluminum round bar-stock and turn down the pin on the end (lathe or chuck it in a drill and use a file) to fit into the seat. This will keep the the tool centered and prevent the seat from cocking at an angle when driving it in..
You want to drive the seats in straight without damaging the machined sealing surface. Try them first, sometimes they fit tightly, other times you can press them in by hand. Depending on how tightly they fit, you might be able to to tap them in using just a wood dowel as the driver.