We will be using Adobe Photoshop for this tutorial.
Section 1 - Preparation
I will be using an image from Google to create the Microsoft Xbox 360 Logo as a custom shape.
I found the image I will be using

Once your all set with the image in Photoshop, you can proceed.
Section 2 - Drawing
1. Select the Pen tool.

and make sure your drawing in pen mode (unless you can free-hand it) and paths is selected.

2. Start making anchor points wherever you like. To Add or subtract from the path, make a new path (if not already done) and copy the whole image and paste in your original work path.

3. Once it's complete move on to the next section.
Section 3 - Shaping
1. Right click on the path and select "Define custom shape" or go to Edit > Define Custom Shape
2. Name it whatever you want and click "OK"
Section 4 - Testing
Make a new image and go to "Custom Shape Tool."
At the top, there should be a drop-down toolbox that shows different shapes, select your custom one.
Draw with it!
Here is my final result:

Here are some others I made:
Windows Logo:

Mac OSX Logo:
