is another great resource on the making of yeast starters. Are they absolutely necessary? That is a subject for debate for some, as many people ferment without them and claim that they have no problems. I am not one of them and always make a starter for every batch. The only time I don't make one is when I am using yeast that was harvested from a previous batch.
FWIW, I don't use an airlock on my starter flask, I just use some sanitized tin foil. The purpose of making a starter is propagating the yeast and that process requires oxygen. Installing an airlock will only allow one half of the gas exchange to take place. It will allow CO2 out, but not allow O2 in.Here
is another, albeit very technical article about yeast propagation.Here
is yet another resource on this subject.