Hi Danni,
If it is a timer with the little clips you need to press down, thenit is easy. Turn the dial so that the arrow points to the time, notrmally works in 15 minute segments. Then press down the little pegs to say when the plug is "on". Just test it with a lamp or something so you can make sure that you have it switched to the timer setting as opposed to the always on setting.
As far as I know the ones with the little pegs, you have 2 coloured pegs. One to show for On and one to show Off. So put the pegs into the "times" when you would like to switch it off. The general just is the same, turn the dial so the arrow points tot he current time.
Just a point to remember though... If the timer isn't plugged into power then it will not keep time.
HTH