A piece of fabric (100% cotton is best)
A Fat Quarter will be enough, 18'' by 20''
A size template for dimensions and measurements
This will save you time and ensure your bow ties are the right size, since it is hard to judge size on the fly, which I learned the hard way. This one here is cut from thin particle-board backing from some old shelves. It measures 5.5cm by 11cm, but you may want to go bigger or smaller. A good way to start is to get measurements from a pre-made one, which is what I did here.
I am using upholstery thread for extra strength, but any thread will do.
Large brass safety pin
I am using a brass safety pin here, but any 2-inch safety pin works. (Brass is just stronger.)
When you have folded your fabric up to a decent thickness, turn it over and look at the back flap of fabric. It needs to run along the center of the folded fabric. You may need to quickly cut it if it does not, or if your fabric is uneven. You need to sew this up, but only along every inch or so. Be sure you are not sewing through the front. It doesn't matter how your sewing looks, since this will be on the inside of your bow tie and won't show when you are finished.