- you will need a yeast cake - so ideally you will just have bottled or transferred your beer to a secondary fermentation vessel
- a large jar - plastic or glass (be careful with glass as the yeast will sometimes still release CO2 once harvested)
- kettle or pot to boil some water
- funnel (to get it into the new fermenter if necessary)
- As with anything that touches your wort (or beer) after the boil, make sure you sanitise everything that will be used for this procedure! Set up a bucket or something with no-rinse sanitiser and dump your funnel and jar(s) in there
- Boil up some water in your kettle/pot to sterilise it. Once it has cooled down you can pour it over the yeast cake, cover the fermenter and shake it around a bit to loosen everything up
- At this point I transfer to a large jar or something, cover it with some cling wrap and let it sit for a good 60-90 minutes to allow everything to settle - this is the "harvesting" process.
- As it settles you will see the layers forming top to bottom: beer/water mix, useable yeast somewhere in the slurry in the middle and then some dead yeast and hop residue right at the bottom. This is the "washing" stage where you separate the yeast from the trub. The idea is to discard the top layer before you pour it over your new wort leaving behind the bottom layer of old hops and dead yeasties (which can also be discarded of course).
- If you plan to store it, you can just seal up the jar or bottle and keep it in the fridge for a couple of days (just ensure to bring to room temp before pitching). It is recommended not to store this for too long, but if you want to get super technical and have a wealth of equipment you can get a bit more out of it according to Wyeast. I prefer to use it the same day or to pitch the following morning.
Always ensure everything is sanitised - even the top rim of the primary fermenter you are pouring the slurry from.
Obviously re-using yeast provides another risky situation where you can transfer or introduce other nasties, so always be careful to keep everything that it will touch super clean and sanitised. The standard iodophor seems to do the trick for me every time.
Harvest your yeast from a primary fermenter.
It is best to obtain your yeast from your primary fermenter (although it is not unheard of to obtain from secondary) as this will ensure you get the most active, flocculent and healthiest of the lot.
Harvest immediately after you syphoned off your beer.
The faster you get the yeast out of there and clean it up, the lower your risk of getting it infected - kind of makes sense.
Try to use the yeast for a similar style.
Needless to say you cannot use your US-05 to suddenly brew a Saison or an English Ale - US-05 is still a standard American Ale yeast, and will always be!
Best to re-use yeast from lower gravity beers on higher gravity beers. Example, you can start off by brewing a simple Pale Ale, next you can use that same yeast to make a slightly stronger IPA and ultimately you can use that same yeast to make that IIPA you have always dreamed of! This way the yeast will be really "hungry" by the time it gets to the IIPA as the previous beers basically serve as starters.
Try to not re-use yeast more than 5 times.
To keep the risk of infection to a minimum, it is advisable to only re-use yeast 3-5 times - unless of course you are a pro and have specific equipment and a team dedicated to taking care of and developing your special strains. For most home brewers, I will safely assume this is not the case...Remember, every time you wash and re-use you expose yourself to some risk so probably best to quit while you're ahead.