Custom shapes work in much the same way as custom patterns. First you make the shape using a standard empty document and using any of the vector based tools or selection tools. This can be the pen tool or any of the custom shape tools. Here's my way of doing it although there are many others.
Creating the shape
You can create shapes from anything. You could use the pen tool, or any of the shape tools. In this example I'm going to take a sketch of an apple and make it a custom shape. First I start by using the Magic Wand tool to trace the outline of the sketch.
Once you have selection right click and choose Make Work Path. This makes your selection into a Path.
If you are not happy with how it has turned out you can use the Direct Selection Tool or the Pen Tool to fine tune corners and edges.
Making it a custom shape
Once you have a path you are happy with pull up the Paths pane. You can find this as a tab in the Layers pane or by going to Window > Paths. You will see your path at the top of the pane. In the bottom left of the pane is a small dark circle. If you hover over it it will say "Fill Path With Foreground Color". If you click on it it will do just that.
Now go to Edit > Define Custom Shape to add this shape into your library. That's it. The shape is now available as a vector shape using the Custom Shape Tool (shortcut U). Having a good library of shapes is invaluable for giving extra touches to designs.
