Press and hold the ISO button on the top left and rotate the command dial on the right until the film speed appears on the display. Release the ISO button and the speed is set.

In case your instructor hasn't told you, IR light is just below visible light on the spectrum and has a longer wavelength. Therefore, IR light will focus behind the film plane for a given setting. I.e., if you preset the lens to focus at 10 feet, the IR light will focus at a shorter distance. Unless your lens has a distance mark for IR, I would limit my shooting to longer distances and smaller f-stops to use the depth-of -field to compensate. When you are in focus for IR, the image in your viewfinder will be out of focus. The closer you are to the subject, the more out of focus the image will appear at the correct focus setting.