Originally Posted by
HannahRage
You can't make a custom brush that will actually squiggle automatically in the iPad app, but you can get a stroke that works like a ballpoint pen so you can draw your own squiggles.
Pick the Ink Pen, set Pressure*, Aspect, Opacity to 100%, turn off Smoothing and Taper Length (or leave Smoothing at 3%ish) and leave Square Head off. You will then have a stroke almost exactly like a ballpoint pen and you can squiggle away.
*If you don't have a pressure sensitive stylus, you can ignore Pressure. Setting it to 100% stops it changing size on you.