i thought this picture might help out some newbies. found it on another forum.
Most starters are made as "1 L" starters, meaning they use a 1L flask or container. Since sizes vary, you may wish to measure more accurately to get the desired propagation of yeast.
Instead of using 1/2 cup of DME, try just using 1/4-1/3 cup to make a lower gravity wort. This helps the yeast get active quickly, and they go through their aerobic stage, eating oxygen and reproducing, rather than producing alcohol.
If you want to be a bit more specific, make a ~1.020 OG starter; use about 2 oz of DME in a final liquid volume (before adding yeast) of 800 mL. This works great in a 1L flask.
You will usually need to top off after your boil with some water to reach the right volume. I just top off with some freshly opened bottled water so it stays relatively sanitary.
Instead of an airlock, use some sanitized aluminum foil. This allows oxygen to get in and feed the yeast.
Swirl the liquid every once in a while to keep those yeast busy. I usually just set it on the counter or desk and shake it every time I pass it. Swirl slowly at first so you don't foam over!
This method can be used the day of brewing. It's best to do it the night before, but getting a starter going in the morning or even a few hours before you brew will ensure that your yeast are awake and ready to rock once you're ready to pitch.
STEPPING IT UP:
There are two basic ways you can pitch a starter:
1. Just pitch the whole thing. This is most beneficial when the yeast are at peak fermentation, happily chugging away and ready for more.
2. Decanting. After your yeast goes for about 24 hours and is finished fermenting, you throw it in the fridge overnight. Then, the day you brew, bring the yeast out, decant (pour out) the liquid, and let it warm up for a few hours before you pitch.
If you want to "step it up" and make more yeast, then I would recommend using decanting, as follows:
1. Make a starter, let's say 1L. So boil 2 oz of DME in two cups water. Top off to 800 mL. Add liquid yeast and foil, and shake for 24 hours.
2. Put starter in fridge overnight.
3. The next morning, remove from fridge and decant. Keep sanitary and let warm to room temperature.
4. Make another larger starter, let's say 2L. So boil 4 oz of DME in 4 cups water. Top off to 1600 mL. Add liquid yeast and foil, and shake for 24 hours.
5. Put starter in fridge overnight.
6. The next morning, remove from fridge and decant. Keep sanitary and let warm to room temperature.
7. Pitch into beer that day or repeat until you have the amount of yeast you want.
If you want to store yeast for a short time, decant and put the yeast into a smaller container so it fills it almost all the way up. Then add an airlock. Make sure everything that touches the yeast is clean and sanitary.
EDIT: This is my new favorite yeast calculator: