I am going to post this topic in hopes that it will be made a sticky and give a basic outline on how to make a yeast starter. If I miss anything or if there is is some additional information I am not apprized too don't hesitate to post or PM me and I'll make changes.
Please note that this is not a post to debate whether or not yeast starters are necessary.
How to make a yeast starter:
A starter is basically just a small batch of beer in which you either "proof/activate" a vial/smack pack of yeast or, in cases of larger starters - grow yeast.
If you are making an average OG beer (1.045-1.060) with a tube/smack pack it is probably a good idea to proof this yeast and give it a running start. If the yeast is fresh you can simply pitch the yeast into a pre-made starter wort the morning of brewday and then pitch this starter active into your cooled, aerated wort. This method will not really grow yeast but it will assure you that your yeast if healthy and give the yeast a "running start". This should also help you have more control over your lag time. If there is no activity over 8 hours or so then you know your yeast was not healthy and a dry yeast pitch may be in order.
If the beer is of a higher gravity (over 1.060) or if your vial/smack pack is old it is important to make your starter 4-7 days before brew day to allow the yeast to grow and settle out of the beer into a slurry. Once the starter is finished fermenting and has fallen to the bottom you should put the starter in the refrigerator until brewday. The cold temps will help the yeast fall to the bottom. Then, on brew day, once your wort is cooled and aerated you can simply take the starter out of the fridge and decant the spent beer and pitch only the slurry. You do not have to let the slurry warm up as there is evidence that pitching cold is actually beneficial.
The starter wort should not be stronger than 1.040 OG (1.030-1.040 is optimal). For a 1L batch of starter you would need approx 3-4oz of DME (12-15oz for a gallon of starter medium approximately). Bring the DME and measured water to a gentle boil (be careful, starter worts love to boil over and make a mess on your stove - be especially careful if using Pyrex Lab Flasks or the like as these create miniature volcanoes of hot wort that can be dangerous - use low heat in this instance and monitor carefully) A pinch of nutrient (such as the Wyeast nutrient) is very beneficial in helping to preserve the integrity of the yeast cell wall.
Once the wort is boiled you should carefully pour it in your starter vessle (if you are using a lab flask you can skip this step as this is your starter vessel) and cover the opening with a piece of aluminum foil (this is sanitary by lab standards) and cool to about 70 degrees before pitching (the lab flask can be immersed in an ice bath to cool quickly).
Once cooled aerate the wort well either by shaking, pumping filtered air or using a shot of pure O2. Pitch yeast.
It's a good idea to pick the starter up and swirl it to re-suspend the yeast and to provide some aeration during the growth/activation period.
The size of your starter should be larger if you need to grow more yeast. For instance, you may need a gallon starter for a barley wine or a 2L starter for an IPA. Lagers and "hybrid ales" such as kolsch will need larger pitches of yeast as compared to ales. In the case of lagers you will need approx 2x the amount of yeast.
Use the Yeast Pitching Calculator
to determine the approximate size starter you will need.
A couple of links of interest
http://www.maltosefalcons.com/tech/MB_R ... turing.php