How to Make a Homebrew Yeast Starter

Totally stolen from Wyeast Laboratories

1/2 Cup Dry Malt Extract (100g, 3.5oz)
1/2 tsp Wyeast Nutrient
1 quart (1L) tap water

Mix DME, nutrient, and water.
Boil 20 minutes to sterilize.
Pour into a sanitized flask or jar with loose lid or foil.
Allow to cool to 70°F.
Add yeast and shake well.

The yeast will multiply for 24-36 hours. If you’re brewing on Saturday evening, start your starter on Thursday evening.

Periodically stir, shake, or otherwise agitate your yeast starter. This helps in several ways.

After 36 hours at room temp, move your yeast starter to a fridge to prevent spoilage before you brew.