When I read about harvesting yeast it looks like a bit of a crap-shoot with a lot of work entailed. You need to find a commercial beer that has live yeast, brew up a mini-batch of beer using 1lbs of DME and trying to culture the yeast from the bottle using fancy chemistry equipment.
I have an easier, much more lazy method.
Step 1) Buy beer that is ‘bottle conditioned’
Step 2) Drink most of the beer (leave the sediment at the bottom)
Step 3) Pour sediment into a glass of Malta.
It’s really that easy. All you need is an empty sanitized jar, a bottle of Malta (found in the ethnic section for under a dollar) and a bottle of beer that has been bottle conditioned (it will have a think layer of yeast sediment on the bottom of the bottle). Drink the beer leaving the yeast sediment behind, pour this into the jar with the Malta drink, cover it up with something breathable (I use a coffee filter and an elastic band) and leave it overnight. The next morning you should see a larger layer of yeast sediment in the jar.
Malta, sanitized jar and a bottle of beer is all you need.
Rogue Brewery doesn’t just make good beer, they own a proprietary yeast called ‘Pacman’. I got a bottle of Sig’ Northwester Ale last night which I am trying to harvest this Pacman yeast from. This morning I can see my jar has a nice layer of yeast at the bottom. I am going to stir it a few times over the morning to try to grow it to a size that I can use in my House Pale Ale, which is still No-Chill cooling this morning. I only have about 4 hours until I am going to need to move it into the fermenter, so if I don’t have a healthy sized yeast sediment by then I will probably just decant the liquid and keep it for another brew.