This is the writeup from our wiki, which isn't bad. The only thing I would do is start with at least a quart sized starter, which is double the size of the directions below. That ought to be good enough (1.5qt might be best for your OG, but don't sweat details like this).
Basically what you're doing is making a small, low gravity, extract only, unhopped beer. If you can make beer, you can make a starter. Just make sure to sanitize everything as usual. Cover the jar, jug, or whatever you use with sanitized foil instead of an airlock - you want some oxygen to get in, but you don't want to risk infection.
Yeast Starter Procedure
Making a starter is relatively straightforward, far easier than brewing an entire batch of beer.
Allow yeast to come to room temperature. If it is a smack pack, smack it first.
Bring one pint of water to a boil in a two quart saucepan on the stove top.
Reduce the heat, add one half cup of Dry Malt Extract (DME) to the water, and mix it thoroughly.
Gently boil the wort for ten minutes.
Remove the saucepan from the stove top, and cool it to room temperature, ~75ºF (~24ºC).
Put the cooled wort into an approximately half gallon (~2L) container.
Cover and aerate thoroughly by shaking or putting it on a stir plate.
Pour the contents of the yeast vial or smack pack into the container.
Shake to mix the yeast into the wort.
Cover the container with a piece of foil or a loose-fitting cap. Place on stir plate at this time if you have one.
There will be very little sign of activity, but the starter should be ready in one to three days.
You may step up the starter by adding additional wort over a period of days.
After you have brewed your full starter, you may either:
Shake the entire mixture again to get all the yeast into suspension and pitch it into the wort.
-OR-
Chill the mixture a few hours to a day before pitching to drop the yeast out of suspension.
Decant off the top layer of liquid before pitching just the yeast into the wort.
