How do you harvest yeast from a commercial beer?

First, make sure there's yeat there to capture! Some people mistake any sediment for yeast. For instance, every German lager I know of is filtered so there won't be any yeast. Assuming there is yeast, make up about 2 cups of 1.020 wort. Flame the opening of the bottle with the yeast you want to capture and pour the sediment into your starter wort. Let it ferment out. You probably won't see much activity at this point. Then you can use 1.035 wort to step it up to the amount you need. Depending on the OG and amount you want to pitch it into it may take a couple steps. And keep in mind that the lower the OG of the beer you want to capture yeast from, the more likely the yeast is to be in good shape.