There are a ton of threads out there, but I'll tell you what I do. It's pretty simple and has been working so far. I do this when I'm bottling or transferring, always off a primary yeast cake. Bring about 40 oz. of water to a boil for about 15 min, then let cool. After you're done bottling, dump this water on the yeast cake and swirl it around. Make sure it's 80 degrees or less or it will kill the yeast. Dump the slurry into a large, sanitized container. I use a pitcher because it pours easy. Wait an hour or so, then dump it into three sanitized beer bottles, stick them with an airlock and put in the fridge. I usually only do three at a time, otherwise my fridge gets overrun. You can do more or less, just make sure everything is sanitized. Some people use jars, but I like to use airlocks so that I know pressure isn't building up. I always make a starter when I use washed yeast, and I've never had a dead batch yet, up to 6 months old.