Summarize the following:
You choose pretty much whatever you like to make the heating pad, but you will want to use a cotton fabric, such as a T-shirt or a pillowcase. Cotton is able to withstand high heats so this is the best choice for your fabric. Ask yourself if what you have chosen would handle being ironed a high heat as a basic guide to it's suitability. Make sure nobody is going to miss whatever you decide to use.
Get your chosen fabric.