Great! How much wort/water do I use? I am using a 2L flask.
You'll have a little bit of a challenge in a 2L flask in that the krausen from your starter may blow out the top of your flask, but you can pitch a nice healthy yeast that will work in high gravity beers.
Here's how I'd do it - target a ratio of 1 cup DME to 1 Qt (4 cups) of water. For a 2L flask, I target a 1.5L starter. In in the process of boiling the wort, you'll have some evaporation so you should start with some extra water to compensate for the evaporation. I'd use 1 1/2 cup DME in 1 3/4 Qt (7 cups) of water, expecting that about a cup will boil off. And while you want to be in the right range, don't stress out about the measurement process. If your wort is a little high or a little low in OG, your starter will still work.
Bring the wort to a boil and boil for 10 minutes or so. Chill the pan of hot wort in a sink of very cold water. Drink a beer and spray your flask and a 4" square of foil with StarSan, then drain and refill the sink with cold water surrounding the pan. Have another beer. When the wort has cooled to the 70* - 80* range pour the wort into your sanitized flask and add your yeast. Cover the flask opening with the sanitized foil and put it on your stir plate (if you don't ahve a stir plate, shake the starter every hour or two to aerate it). Cover the surrounding area with some paper towels in case you have a krausen explosion. Look at the starter the next morning and you should see some activity. The second morning the starter should be really happy.
That evening when you first get home, put the flask in the fridge to crash the yeast. After dinner repeat the wort boiling/chilling process. Pour off the liquid from the flask, leaving the thick yeast in the bottom of the flask in tact - you'll need to leave a little of the liquid in the flask because you'll loose too much yeast if you pour it all off. Then add your second batch of cooled wort to the flask and set it back on the stir plate. Check on your yeast friends the next two mornings. The night before brew day, put the flask in the fridge to crash the yeast, and during the mash, pull the flask out, dump off the liquid and let the yeast warm up to room temperature.
If something happens and you can't brew, your yeast will be good for a few months as long as you keep it refrigerated and covered.
Best of Luck!
