If you want to screen an 8-foot bathroom window, gardenias won't do it any time soon. Do some research to see to what size shrubs will grow and how long it will take them to reach their maximum height and width. Once you nail down how you want it to grow, you can start narrowing down your options. How much pruning are you willing to do? Formal-looking shrubs like boxwood and yew require regular shearing. Oleander and laurels are better looking if you hand-prune them. Natural, low-maintenance plants are junipers and dogwoods. Don't pick something that requires more work than you're willing to put in. Think about what you want it to look like all year long. Some shrubs are really showy for a season but you might not like the way they look the rest of the year.
Create an outline for your shrub by marking two parallel lines (or curved if you want a curved hedge) down the path you want your hedge to cover. You can use an inverted tip spray paint, or a can that allows you to spray upside down, to mark the line. If you have a straight hedge, you could also drive wooden stakes down into the soil every 4 or 5 feet and tie string around these to create a really visual outline. The two lines should be the same distance apart as the diameter of the shrubs' containers, or the size of each shrub's root ball. This will allow you to visualize the hedge in your landscape and it will give you a planting guideline so your hedge will follow a straight (or uniformly curved) line.
If you have grass or other plants in the area between your lines, remove them so your new shrubs don't have to compete with them for water.
Set the plants out (while they're still in their pots) in the spaces where you'll plant them. Don't start them off too close together because they'll need room to fill in. If they're too heavy to move, place markers like pinecones out instead. Step back and look at the hedge, moving plants around that are too close to or too far apart from the plants next to them.
Move the plant out of its space in the line and dig the planting hole. The hole should be wider than the root ball and almost as deep. Measure it against your shovel and don't dig down further than that. If you dig too deep, you'll have to backfill with loose soil and the plant will sink too deep.
Set the root ball in the planting hole, making sure your plant sits just above the soil level. Once it's watered in, it will sink a little bit but you don't want it to go too low. Backfill around your shrub, adding any amendments and fertilizers the plant needs. Do this for every plant down the line.
Once all the shrubs are in the ground, you'll need to water them. An easy way to irrigate if you don't have a sprinkler system is to lay a soaker hose along the front of the shrub and loop it around the back. Give your plants plenty of water at first and water them regularly, according to each shrub's requirements.
Finally, add a 2- to 3-inch layer of mulch on top of the soil around your plants. This can go right over the soaker hose. It will block out weeds and keep your plants moist.