Hedges should be trimmed 2 - 3 times a year unless you have a few types that grow quicker than the average hedge in which case 4 times would be more suitable. You have to be careful with how much you do trim your hedge as you are basically amputating which the hedge can most certainly have a bad reaction to.
Use taut lines attached to canes in order to know exactly where you are cutting. Once you get some experience you will be able to do this by eye but if you are a starter it always good to have these lines to ease you into hedge trimming.
Cutting the side of the hedges with a vertical sweeping motion as you walk along the hedge is the standard technique that most gardeners employ. Cut the top of the hedges with a horizontal sweeping motion.