It is a bit different to take care of than shorter styles. One of the things that you can do at first is find a photo or video of a long hair style that you or your client or friend likes. Do not try to cut complicated styles at first if you have no experience.
A head of long hair is more easily cut when it is dry. This is a bit different from other styles that require dampness. This will also make sharp scissors or shears dull, but it is one trade-off of having long hair. The day that the hair is to be cut it should be washed with shampoo and conditioner. Dry it normally. There should be no chemicals added to the hair before the cut takes place. You will need more tools to cut it, such as a fine tooth comb, and scissors or shears designed specifically to cut hair. Do not try to use regular scissors.
The first step in learning how to cut long hair is to use the fine tooth comb to get out any knots. Since you should have already decided what style you want, now is the time to determine what the length will be. Take care to cut off less than you actually need at first. Cutting the hair perfectly across horizontally is the first goal. This is achieved by bending at the waist and allowing the hair to point to the floor. Cut straight across horizontally as straight as possible. To check your work, stand the person ( or you if you are the one getting the hair cut ) up and see how even the cut was. If you did well, great, continue cutting until you reach your desired length.
The other steps in learning how to cut long hair involve framing the face correctly. You can cut a V-shape by starting layers. This involves tilting the head forward or backward and cutting to the desired length. When you want to check to see if the sides are even, bring up hair from both sides of the head and compare them directly above the head.
Source : http://ezinearticles.com/?How-to-Cut-Long-Hair&id=2809459