When planting, there are a few important things to consider; equal (or close to) sized plants and soil quality. Having similar sized plants will ensure they grow at a similar rate. Soil quality is very important. When growing hedges, you will get good healthy growth in a good quality soil.
Line the area of planned hedging and dig up about 1-2 feet deep and wide. It is also a good idea to mix in compost as this will supply the plants with vital nutrients. Small hedges like Buxus should be planted 20-30cm apart. Larger hedges should be planted between 50-100cm apart. When buying the hedge, ask the salesperson for distance between your species of hedging.
Look at my post on How to Plant a Shrub or Tree for more detailed information on the planting process. Best of luck with your hedge!