If you look under the hanging cabinets in your kitchen you will notice that they have a recessed space about an inch deep. This space can easily accommodate the disk or "puck" low-voltage lighting fixtures that are available at most lighting stores. Before you go shopping, though, make a rough sketch of your cabinets and counters, including the height and width of each cabinet. This helps you determine exactly how many fixtures are required, as well as how much wire is required for installation.
Most lighting stores have low-voltage under cabinet lighting kits that use halogen or xenon bulbs. Both work the same way. They come with a transformer that converts 120-volt household electricity to the lower voltage used by the low-voltage fixtures. The fixtures are connected to each other in a series, with the last (or first, depending on your perspective), connecting to the transformer. These kits generally come with three or four fixtures and are expandable up to six or 12 fixtures, depending on the wattage of the fixtures and the size of the transformer. Do not exceed the maximum output of the transformer (for example, a 125 watt transformer can handle no more than 10 12-watt fixtures), because this is a fire hazard.
For ease of installation, plan on buying at least one transformer for each connected set of cabinets.
Your lighting kit's instructions explain exactly how to install the lights in a series, but in general you have two wires running from the transformer to the first light fixture and two wires running from each light fixture to the next fixture in the series. You string the wires through the back of your cabinets. Make installation easier by pre-drilling the holes through the bottoms and sides of your cabinets, and measuring, cutting, and running the wire you need. Now install the fixtures one at a time. First attach the wires to the fixture per manufacturer's instructions, screw the fixture to the bottom of the cabinet, then move on to the next in the series. Next, install the light bulbs, and finally plug the transformer in to an outlet and you have light!
Source: Ask the Builder