The best time to prune is about now or in the fall when the tree is dormant. You should be taking off 15-20% of branches for young trees or 30% for older trees. If you prune too much off, the plant sets off these things called “suckers” which come out of the ground and resemble a stem. This happens because the roots are now relatively more active than the top growth, and the energy has nowhere to go, so suckers are created; which are counter-productive. Suckers may also appear if the tree is stressed, as you can see in this picture even before I pruned.
Pruners or clippers are used for small branches and saws or large shears are used for larger branches. Some say cutting on an angle (45 degrees) is better for shedding water. Others say cutting straight on (90 degrees) leaves a smaller surface area for diseases to infect. I choose straight on but we'll see how that goes. I was told at the seminar that you should cut low down to the collar of the branch while other people I have talked to said to leave a 1/4'' of branch left. I chose to do some of each. The picture below is a spur, a short stubby twig that will flower and produce fruit. It is recommended to prune spurs for larger fruit and to produce more spurs. Also remove any that are too close together or diseased.
Types of branches you should be cutting of are ones that are criss-crossing, broken, diseased, downward/upward growing or are making narrow crotches. Apples trees also tend to have a dominant stem right in the middle that all of the other branches come out of. This is called having a central leader. Some people may keep it this way but others may strive to have an open centre. This allows for more air-flow, adequate light to hit all of the leaves and a higher yielding, more productive tree. In some trees, I kept the central leader because it was hard to remove without removing a large portion of the tree but in my older one (seen below) I strived to make an open centre.
I have never pruned these trees before so I pruned them very hard. I think I may have pruned too much but they say it’s difficult to prune an apple tree too hard. I'll let you know what I did wrong later but here are my before and after pictures of the one tree. Sorry I rambled on a bit but it turns out that pruning trees in general is sort of an art and like most arts, it takes a lot of practice to get good. Just remember to stand back once in a while and look at the bigger picture.