Tips and techniques
There is no need to purchase paint specifically designed for fabric. You can buy fabric medium and simply add to your acrylic paints. There are several brands to choose from. (I use Liquitex but there are many others available) The medium will help with the paint adhesion and allow the fabric stay soft.
It is a good idea to wash a new fabric item before painting so as to remove any sizing, although I have painted on new items successfully without washing in the past.
Map out or draw your design on your fabric item with a pastel chalk pencil so if you make a mistake you can easily brush it away. You can use iron on stencils but be aware if you have a dark item you may need to get one that will show up.
You can also transfer a basic pattern onto the fabric item by tracing your design on a sheet of tracing paper. Turn the image over and go over the lines with the chalk pencil. Put the chalk side on your fabric and rub with your finger around each line to transfer. This method will not work for intricate designs but will allow for proper placement and a basic image.
You will need a stiff brush for the initial lay down of the base coat. Make sure you scrub the paint into the fibers of the fabric.
Before painting attach your fabric item to a stiff board. You can buy t-shirt forms that will fit clothing items or just use some heavy corrugated cardboard wrapped with waxed paper. Good sources for large flat pieces of cardboard are wholesale clubs. The cardboard is usually in a bin and free for the taking.
It is also a good idea to heat set your painting after it is finished. Simply place piece of fabric over the image and run a medium heat iron over it.