Question: How do I make a "starter"?

Answer: A "starter" can be any volume of wort that you add yeast to before using it to make your beer. The yeast get active in this smaller volume, usually for 1-2 days, and then can be added to 5 gallons of beer, or 10 gallons, or whatever size your brewing. This can be a good way to "proof" the yeast, and also when making high gravity beers. White Labs recommends on their label to make a starter "if the gravity is over 1.070, if the yeast is past its "best before" date, or if a faster start is desired.

Procedure:  

Make up a 1-2 pint wort, gravity ~1.040, hopped as normal. Boil for 30 minutes, cool to room temperature. Pitch one vial, shake well and let sit for 1-2 days.  Little to no activity will be seen in the starter, since this is a very small volume compared to the quantity of yeast pitched.  The yeast in a starter may be done within a couple of hours. But a layer of yeast should be at the bottom after 1-2 days. The wort on top of the yeast can be either decanted of the top, or left in and pitched with the whole volume. Most pitch the whole volume, but if the starter gets to the point of 2 liters for 5 gallons, then we recommend decanting the wort off the yeast.
Typical Starter Volumes for 5 gallons:
To activate the yeast: 1 pint
To regenerate expired yeast (there will be living yeast in the package for ~1 year): 2 pints
To brew a high gravity beer: 2 pints
To brew a lager beer, starting fermentation 50-55F: 4 pints