How to prune cordon tomatoes.

When growing tomatoes you want to get the maximum yield of crops with the best flavour you can. Pruning your plants to remove unwanted shoots and leaves is an essential part of this. 
Step 1: When to de-shoot
The aim when growing a tomato plant is to have a single stem. Wait until four or five fruit bearing trusses have grown from the stem, then pinch out any additional side shoots leaving the plant's top shoot, known as the terminal shoot.

Step 2: Pinching out
Grab a growing tip by the base between the thumb and forefinger, and bend it back and forth until it snaps cleanly. This should ideally be done when the shoot is young and supple. This should ensure that the majority of the nutrients are sent to the fruits, instead of being wasted on the unwanted growing tips. Avoid cutting with a blade as the wound can become easily infected, although this may be easier on older, tougher shoots.

Step 3: Removing leaves
As the plant begins to mature, the lower leaves will naturally begin to yellow and wilt. This is perfectly normal, so pull these from the plant when they appear. It will keep the plant fresh, looking good and help ward of disease.
Step 4: Topping
To get the best out of the last growth of the season, it is necessary to "top" the plant. About a month before the first expected frost, or when the plant hits the roof of your greenhouse, remove the plants terminal shoot. At this point in the season, the tomatoes currently growing will have a limited time to reach maturity, so all nutrients must be directed straight to the fruit.

It is very important to trim off all the shoots off  of Cordon type tomatoes. I have found a very good video which will explain it far better than I could . Like they say, you can pinch out most shoots by hand and only use a knife for the very large ones that you might have missed.
Pruning tomatoes video

This picture will help you workout which are the shoots on a very overgrown plant.
If you notice, the main stem has larger trusses on it. The suckers are shoots that grow from the bottom of the plant. All shoots and suckers should be removed as small as possible.

This is a tomato truss which grows off of the main stem, do not ever remove it. They will also grow on shoots, but you should not really let shoots grow so big that they develop trusses on cordon tomatoes.