Once you have a radio controller for your flash, you put it in the hot shot mount on top of your camera, and turn it on. Now, turn on your flash unit, press the test button on your controller, and the flash should fire (sometimes the test button is the power on button, and on some flashes, the test button is backlit, to help you find it in the dark). If you press the test button and it doesn’t fire, here’s what to check: (1) As “duh” as this sounds, make sure both the flash and the controller have the power turned on. It’s easy to forget to turn one or the other on, or to forget you turned one off at some point. (2) Make sure your controller is set to On for that flash. Since you can turn flashes on/off temporarily from this controller, it’s possible that you have your flash turned off on the controller (you’d still see the power light on the flash unit itself, but if you have that flash set to Off temporarily on the controller, it won’t fire). (3) Check to see if both the flash and the controller are set to the same channel (in short, they have to be on the same channel, or it won’t fire the flash). (4) Check to see if your flash controller is fully seated in the hot shoe mount on the top of your camera (technically, the flash should still fire using the test button whether it’s fully seated or not, but this is just a good thing to check once the shoot is going and your flash stops—good chance it’s not fully seated if that happens).

Excerpted from The Flash Book: How to fall hopelessly in love with your flash, and finally start taking the type of images you bought it for in the first place, by Scott Kelby