Reusing yeast is very easy. After you let the yeast settle out of your beer, rack the beer off into your bottling bucket or keg, leaving just a tiny bit of beer behind with the yeast. Swirl that around so it mixes with the yeast and breaks up the cake, then pour it into a sanitized canning jar and put the lid on it. Store in the fridge. If you brew another beer within a couple weeks of harvesting your yeast, you can simply pour the right amount directly from you jar into the new wort. If both beers are at about the same gravity, you will want to pour about 1/3rd the yeast in the jar. If the new beer is of a reasonably high gravity, then 1/2 to 2/3rds of the yeast should be used, and for a barley wine or similar you can use all the yeast.
If you want to use the yeast more than a month after you harvested it, you should make a starter.