Starters are really optional
Let me see how do you say this um?!? NO! For liquid yeast you are playing Russian Roulette with stuck brews, off flavors and higher fusel alcohol content. Just scan through the posts here and look at the people asking about stuck brews and high finishing gravities I will bet they did not pitch correct numbers and/or aerate.
Dry yeast is a different story, it is usually easier and cheaper to just pitch another packet to boost your cell count to the correct level and also dry yeast are dried out when really healthy so they rehydrate good to go.
There are a few key concepts for yeast dry or liquid.
1) Cell count. This is how many yeast cells are required per ml of wort. You can calculate it or use a tool like Mr Malty. This will tell you how many Billions Yeast cells you will need to ferment your beer correctly. No enough yeast creates yeast stress and they create different esters and more fusel alcohols and also leave more sugar unfermented in your beer. Too many yeast cells (rarely a problem unless you pitch on to a previously used yeast cake) will cause less esters to be produced and as the majority of yeast flavors are created by the cell reproduction, so it is not like it is as bad as underpitching but undesirable none the less. Mr Malty is great for beginners as it will tell you how to turn your vial or smack pack into enough yeast to make your brew. It also will tell you how much dry yeast you will need if that is your choice.
2) Viability. This summer has been really tough on people all over getting yeast that has got hot in transit. This is typical of what can happen your yeast and neither you or your LHBS can tell you exactly what the condition of your yeast is when you buy it. So making a starter confirms that the yeast you are using is alive and that their is a good number of them. If you yeast starter takes an extended amount of time to take off then there was something wrong up 72 hrs is OK but not ideal. If you have a starter with massive lag times chances are it got a bit knocked around in transit or is old, consider crash cooling decanting the used wort off the yeast cake on the bottom and add more wort (this is stepping up a starter).
3) Cell health. Having happy yeast cells that a fat (ie cell walls are nice and thick that can withstand high sugar/alcohol content) Will make you yeast perform well in their task of fermenting our precious amber nectar. What makes healthy yeast... O2 + Nutrient + Sugar. This is why some people use stir plates as they continuously provide gas exchange for the yeast during the starter. Other people blow pure O2 into the starter with a SS stone and other shake it like a Margarita as often as they can. Also consider adding a pinch of yeast nutrient to your starters and also to your wort (dosing for the wort as recommended by manufacturer). O2 for your wort before pitching is also a key to avoiding stuck brews and off flavors.
3) Pitch temperature and fermentation temp control is a major issue for the wort but not really for your starter just do it around 70-75ish (note the starter will smell and taste like really stale beer without hops... yuk but it is normal).
Making a basic starter is as described in the link BainbridgeBrewer posted just make sure you are adding lots of O2 and nutrients and the sugar content of the wort is not too high, not greater than 1.040 10grams DME per 100ml of liquid is a rough rule of thumb. If you want to get more advanced check out making a stir plate and using a sanitized piece of foil for a lid.
Things I would add is plan on making your starter and crash cooling in the fridge (this is where foil is better than airlocks, less room required) for 24 hrs and then decant leaving a 1/2' of wort on the yeast cake (to help mixing) at the bottom of the starter flask. If the starter was successful you should see yeast + trub. The trub is the brown stuff on the bottom and the yeast is the white stuff, this is called your yeast cake. You should see more yeast than when you pitched if your starter worked. Now some people swirl the flask with the yeast and pitch it straight into the fermenter, I like to do it a bit different, I decant the morning of the brew and then add the decanted starter to a second starter and pitch at high krausen. I tried to time the initial starters to be at high krausen when I first started making starters, however I could never predict the lag time (or brew time around kids nap schedules) so I started making the second starter to allow me the flexibility. It is point of discussion on when to pitch the starter, you will develop your own process that works for you.
I'm a self confessed yeast nut so bare that in mind when reading this however yeast is arguably the most critical component in making good beer.
Hope I did not scare you off with the long post but if you follow the steps of Mr Malty and BainbridgeBrewer link you will be fine
Clem
