I started cropping and repitching from my third batch ever. It is not hard at all, actually results in better beer, saves money, and is kinda fun. You get to use flasks and pour stuff back and forth and rub your chin and look wise.
This article from the Wyeast people is geared toward commercial breweries, but I learned a lot about cropping from it.
I basically followed the steps here, but with some of my own tweaks. I also used the invaluable pitching rate calculator at MrMalty.com.
Save some slurry from primary, thin it with water, let settle in fridge.
Discard the top third (water), save the middle third (yeast) and discard the bottom third (trub).
The saved slurry is still a little thin for storage. Let it settle in the fridge overnight, then discard all but a bit of water, then swirl the rest so that it's pourable into a clear bottle for long-term storage.
Well before you rack, boil some water and let it cool to room temperature. You use boiled cooled water not just for sanitation, but to keep the yeast dormant by depriving them of oxygen.
Here's how I did it at racking time (keep boiled cooled water on hand):
- Rack from primary.
- Gently swirl the tiny bit of beer left to break apart the yeast cake and mix it into a slurry.
- Pour a generous amount of slurry into a sanitized straight-sided jar or beaker (not an Erlenmeyer flask... see below). Don't worry if you get hop remnants in there. We're going to leave them behind.
- Let the jar settle in the fridge while you clean your carboy.
- Discard the top layer of water, pour about half the remaining stuff into a sanitized Erlenmeyer flask (or just another jar).
- Top up the flask to about halfway with cold water, swirl, and let sit for a good half-hour in the fridge.
- Pour off almost all the liquid to get as thick a slurry as possible, swirl to mix the rest with the settled yeast, pour as much of the slurry as will fit into a sanitized clear glass 12 oz. bottle. Cap this bottle. (You want the clear glass to assess yeast color. You don't have to worry about light-struck flavors here as all the hop compounds which get skunky have hopefully been washed away.)
- Label with yeast strain, generation, and the cropping date. Keep in the back of the fridge for up to a month.
I don't use Erlenmeyer flasks for separating out the middle third of the slurry because they're very good at pouring a cross-section of the entire contents of the flask. That's what we don't want for the first step. We want to be able to control which "slice" of the yeast we get, so we avoid both water and trub. Beakers or straight-sided jars are better for this.
If you're using it within four weeks, assuming you get about a half-bottle of settled-out yeast, pitching just the slurry should work for beers up to about 1.060, assuming a 5 gallon batch.
- Pour your next batch down on your yeast cake. Others will tell you this is a bad idea, because it's overpitching and will affect the esters and head retention. I tested that hypothesis by intentionally overpitching, and found no evidence of muted esters or poor head retention. I make excellent beer regularly with this method, and it's easy.
- Use dried yeast. Seriously, it's cheap, makes wonderful beer, and you can keep a bunch of varieties around for years. Why go through all these steps when you don't have to? A huge number of commercial brewers use dried yeast.