Basic Skill Building - Squirkling

I'm working on building some basic skills through the help of Drawspace.com If you are interested in drawing skills, you should check it out.

I need a lot of help on shading and providing texture. One of the basic methods of shading taught is "squirkling". It's using random squiggles/circles to provide an organic looking method of shading. Here's a tree on a cliff and a sketch of an iris and an avocado showing this shading/texture method.