If you use the Queen "spawn larva" ability, you have every time after they spawn seven larva. select the hatchery and select larva. then, click in the beginning once V for overlord, then build the rest, type Z for zerglings, etc. so you won't forget. (V, Z * 6 or V, R * 6).
if you build heavy supply units, like ultralisks, press V twice. this way you will never be supply blocked. (if you run low on minerals, or if you see you have a lot of supply unused, stop building overlords for a while).
you have to build your overlord first, because if you have used all your larva, you will not be able to build the overlord right away, but to wait until a new larva spawns. and that takes a lot of time.