Lock-picking is a manipulation of a lock components to unlock it with no key commonly by using two main tools:
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Pick - Pick is a long pieces of metal that curve up at the end, thin enough to enter the lock keyhole. Picke are used to reach into the lock cylinder and lift up the pins. 
Tension wrench - Come in variety of sizes and shapes. plat piece of metal with 90 degree angle at the top and a thin flathead screwdriver alike head to enter the lock and put pressure to the open side.
The first stage is to insert the tension wrench into the lock keyhole and push with the finger to the same direction that the key turn to open. The plug turns so that it is slightly offset from the cylinder around it which produce a little ledge in the pin shafts.
In the second stage, while employing force on the plug, the pick is inserted into the keyhole and start lifting the pins. The goal is to lift each one of the pins up to the exact level of at which it's edge moves exactly into the ledge created earlier by the tension wrench, as if pushed by the match key itself. (should be a slight click sound with every pin falls into position)
The ledge hold the pin in the housing keeping it from falling back down. At this point, each upper pin matched into the correct position in the housing while all the lower pins fall inside the plug and the plug rotates freely and you can open the lock.
Basically, the process of lock-picking is quite simple, but is skill that is a very difficult to master. Locksmiths have to practice the coordination and how to apply exactly the right pressure and what sounds to look for. In addition, they should visualize all the components inside the lock and the lock's design.