Pruning know how
There have been many books written on this subject,
but a little common sense will formulate a volume in your mind.
Since pruning is not an especially enjoyable undertaking, you must have a reason or incentive to tackle the job. Pruning is done for different reasons. You may want to increase flowers or increase fruiting or create a specimen plant or make a hedge look like a Volkswagen or a Puppy; See Precision Pruning.
Injury or Disease -
The number one reason to prune is probably injury or disease. If a big limb breaks off and lands on your roof, get out the chain saw and ladder, and do it now. However, this emergency action may have been avoided if you had pinched the limb off with your thumbnail when the limb was still a bud on the tree trunk.
Potential injury or disease -
Any dead or injured or diseased wood should be removed First. Then any limbs that may become a future problem are pruned next. A bud that is aimed at your house door should be "Nipped in the bud". A weak crotch is grown when a limb comes from the main trunk at a sharp angle ( both growing toward the sky ). A strong limb grows straight out the side and makes a perfect place to hang a swing.
A weak limb may be encouraged to grow more horizontal by roping it to a lower limb or a stake in the ground. Once a month, shorten the rope so the limb is slowly pulled toward the ground. As it grows, it will take on a more horizontal posture.
Any limbs that grow toward the center of a tree or shrub are potential problems. They not only limit sunshine and air from minimizing disease and insect damage, but there is also a good chance that they will rub each other or narby limbs. These are easily pruned off when they first appear, but become difficult when they are mature and crowded.
Suckers are another problem. They can be identified by their habit of growing fast, and straight , and straight up. They may grow from the trunk or the root area. If they are low on a tree, shrub, or rose bush, they may be sprouting below a graft. A grafted plant has two general parts; the roots and the trunk, joined by the graft.
The graft area usually has an unusual look about it; like a healed wound or growth or scarred area. Suckers that grow from below a graft will take nourishmant from the plant and not produce the same fruit or flowers as the grafted plant. They also tend to replace the grafted trunk. They need to be removed whenever you see them.
Fruit or Flower production
Generally speaking, plants need circulation of air and penetration of sunshine to produce the best Fruit or Blooms. So the tree should be opened enough to allow this to all areas.
Most trees and plants grow in an outward and upward pattern, mostly determined by sunshine. That pattern can be altered by proper pruning. There will be little growth toward the shaded side, but much growth toward the side that gets sunshine. Limb buds and leaf buds and flower buds need warmth to develop. Usually that equates to sunshine.
If your desire is for a global shape
If, for instance, you want a hedge to look like a Sherman Tank, it will be necessary to start when it is young and give it monthly prunings that will remove the new growth coming from areas where it is not intended. Allow it to grow only into the area that will be within the tank shape. Cut away everything that does not look like a tank : )
Accessories like the cannon must be planned in advance, since a single limb or two growing at the angle of the cannon will provide the best appearance
Eventually all of the tank shape will be filled in and additional pruning will cause new growth to come from the sides of the old growth since it is not allowed to grow from the tips or ends of existing growth. Then the tank will begin to take on a more solid look, and you will have accomplished your goal.
You will want to select plant varieties that are popular in your area for your pruning artistry. And you may want to construct a wire form tocontain or control the growth.
Actually, you could make a form that resembles a tank, and then allow three inches of growth past all areas of the form. This would automaticall produce a tank shape.
Ropes or wires can also be used to train new growth into intended areas.