There is a small triangle in the top right hand corner of the brush pallette.Click on it you will get a drop down menu including the option "Define Brush".I think it is greyed out unless you have already selected an area ready to make into a brush.I use the circle selection and just use one of the preloaded brushes to gauge the size I want.I create the brush black on white.Once you have created the brush and it appears in your pallette,double click on it to get an option called spacing.This contols the area between the brush marks as you drag it along.A low setting gives you a continous line (good for hair,wood grain etc) a high setting gives a series of patterns one after the other.