These steps are roughly the same for a plant of any size. 1. Dig a hole about as deep, and about twice as big in diameter as the pot the plant is in. If soil is very hard, dig a little deeper, but backfill with soft dirt or potting soil so that the plant will sit at the same level that it was at in the pot; don't plant it any deeper (unless it's a tomato). 2. Carefully remove the plant from the pot. If the rootball is mostly soil, go ahead and plant it. If there are mostly roots, coiling around the pot and all bunched together, you should gently try to stretch them out and separate them. 3. Put the rootball into the hole (plant should be pointing up ;o), spreading the roots out in the hole. 4. Backfill with dirt or dirt/potting soil mix. Water well to settle the soil. 5. Water frequently for the first 2 weeks, then less often thereafter. Watering deeply but not as frequently will help the plant root more deeply instead of having it depend on surface roots, so the plant will be more drought-tolerant.

Here are some added notes about transplanting. You may have noticed that sometimes the transplant does not make it. A lot of stress is involved when a plant changes its environment. Soil, temperature, amount of light, "neighbors", humidity, etc. It is even an adjustment when people, themselves, suddenly switch to a new location and new way of doing things...you probably recall times when you moved and the stress involved. For plants it is a similar stressfull situation. Get some liquified seaweed. Liquid kelp is the same thing. This is an amazing nutrient substance and has all kinds of properties which benefit the soil and plants. It even deters disease and insects. Mix about two ounces per gallon of water. Use this mix of water and 2 ounces of seaweed to soak the roots of your plant (bush, tree, vegetable, seedling, even seeds) for about an hour (longer is certainly okay). Use some of this liquid mix to wet the hole where you are going to place your new plant. It shocks a plant for its roots to suddenly hit dry soil, so place wet to wet. Except for tomato plants, try to have some of the root flare showing, especially and emphatically trees, and never let the base of the stem of the plant be covered with soil. Many pots from the store or nursery often have soil around the base -- remove it. This can cause fungal problems. Vines are particularly difficult to transplant successfully. That is, watermelon or gourds, etc. have a hard time with it and prefer to start from seed at their spot. Unfortunately, most nurseries load their plants with chemicals and synthetic fertilizers to keep them "pumped green", like steroids. This actually weakens the plant and soil in the container. It opens the plant towards diseases and problems with insects. So, preparing a natural and healthy soil for your new transplant can go a long ways. You can upset the natural healthy balance of good soil once any type of synthetic chemical is used...and other healthy, natural applications will be less effective. For trees, don't do a thing to the soil in the hole -- no peat moss, no potting soil, no fertilizer, nada. Use the original soil to backfill. This promotes the tree to not "group" its roots around one specific area, but rather to extend and become acclimated to the new environment. Always dig a jagged, rough cut hole about three times wider than the ball of the tree and never deeper than the pot. You want the top roots, the flare, exposed on all trees, in order to maintain a healthy tree and not stress it. For other plants, such as your garden flowers or vegetables; potted plants and herbs; roses, azaleas, hydrageas, violets; there are natural amendments which you can add to your soil in order to make a healthy environment for your plant. Some things could include a touch of molasses, greensand and rock powders, cornmeal, and certainly compost. Mulching to top of the soil of any transplant with a little compost helps. Add some cornmeal for fungal control and to promote beneficial micro-organisms. Lava sand applied on top of the soil can really help both potted and ground planted plants. Mulch the area around your larger or more established trees. Try to use a mulch which is native to your area. Never that poisonous colored mulch. These are a few suggestions when transplanting plants. P.S. Ha! You will laugh at this one: Talk to your plants when transplanting them...or caring for them. It won't hurt, "unless someone is watching" ...ha! Actually, tests have been done. It can be very effective.

The other answers are both very good, but I wanted to raise one additional issue - particularly for trees and shrubs that you are planting in your yard. There has been an ongoing debate as long as I can remember, about whether it is beneficial to a tree to amend the soil in the hole prior to planting the tree. The arguments go something like this: 1. If you do not have good soil (soil rich in nutrients), you will "starve" the tree by putting it in without amending the soil. All plants need some basic ingredients (NPK and trace elements) that are, unfortunately, not present in all of our soils. The best way to plant a tree, is to dig a hole that is twice the diameter and twice the depth of the container. Then put amended soil (compost added) in the hole to raise the tree up so that you do not bury the crown. Fill in around the tree's roots with more amended soil. Water well for the first two to four weeks after planting. 2. Under no circumstance should you amend the soil when planting a tree or shrub. Doing so will encourage the plant to grow vigorously within the amended soil, and it will not even attempt to root out into the surrounding "poor" soil. This can cause teh tree to become root-bound, much the same as if it is in a pot. The best method for planting is to dig a hole twice the diameter of the pot and the same depth as the pot. Use a spading fork to loosen and aerate the soil in the bottom of the hole. Plant the tree so the crown is at or slightly above ground level. Water well for two to four weeks after planting. If you have concern about whether the soil contains sufficient nutrients, apply fertilizer in the form of "tree spikes" or a topical fetilizer. Now for the right way......choose the method that appeals to you. This debate has been going back and forth for decades...maybe even centuries! I have tried both methods, and have had success with both of them. In fact, I have three weeping willow trees in my backyard that I planted five years ago. One of them had amended soil put into the planting hole, and two didn't. These trees are all now twenty feet tall, and have six+ inch diameter trunks. Not bad since they were as thick as my thumb and less than six feet tall when I planted them. (Of course, I'm sure that their growth has been significantly contributed to by the fact that I watered them with the "waste" from my fish pond filter for the first two years after I planted them!)