Find experienced and qualified professional for hair extensions that is well established in this field. Ask him for a fee, to teach you this craft. A few lessons would be enough. If you are not able to find the person for trial lessons, buy a mannequin with hair. Because there are many building technology ("hot" and "cold"), discuss in advance with the hairdresser which one you wish to master.
A little more expensive than individual lessons of the master, will cost you professional courses on hair extensions. If financial capabilities allow, in parallel, learn and hairdressing. Read all types of extensions and select the best option for yourself. Find out how to care for artificial strands. The combination of theory and practice will give good results and you will be able to avoid many mistakes in the future.
Become a Guinea pig. To do this, look for sites or find out from friends that require models which are practiced novice professionals. Call beauty salons, hair salons, training courses. First, you don't have to pay for the training, and secondly, you will be able to see, as is the procedure, and be able to perform all the actions themselves.
Try to increase the hair on their own, without the help of professionals. Here, however, you can hardly avoid making errors. For the beginning to professional shops to hairdressers will buy a natural or artificial strands and additional materials (metal beads, sticky tape, etc.).
Find online video tutorials, which will be your good example. Don't forget about the possible risk, as improperly performed procedure can result in baldness. Because it's better to train on the dummy. Learning to increase hair myself, and well having mastered this art, will offer a similar service to your relatives or friends. The more you practice, the more experience and potential customers. Perhaps in the future it will become your main profession, which will bring you considerable income.