Timers are generally digital, electromechanical or electrical. Timers can control swimming pools, air-conditioning systems or simple household lamps, among many other things. Mechanical timers can be wound---common in in-wall applications---for controlling a heat lamp in a bathroom, for example. Advanced timers include astronomic timers for street-lighting. Next-generation timers can be programmed with up to 4,000 on/off events and can take advantage of eight-year batteries.
Plug a device into the timer and switch the device on.
Take a look at the timer's dial, and look at the green and red trippers. They are small pieces of plastic which trip the circuit on or off as the timer wheel advances. Some timers have them built-in and some have them loose, in which case insert them into the dial.
Insert a green tripper at the time you'd like the device to turn on. Or, in the case of a timer where the trippers are built-in, push the tripper out from the centre of the dial and slide the tripper around the dial to the time you want the device to turn on. Then push the tripper to engage.
Repeat with the red trippers, which turn the device off.
Set the time. White in the small window indicates a.m.; black indicated p.m.
Plug the timer into the wall and rotate the manual override switch until the device turns off or on, depending on your requirement for that time of day.