There are a variety of different techniques involved in tattooing, many divided between three major areas – lining, shading, and color. Each requires it's own form of application and process and every artist approaches it differently as well as having their own preference in equipment. When learning to tattoo most people simply will start by following their mentor, after they have become confident in creating a simple clean tattoo then they can start branching out and looking at more advanced techniques like whip shading, blood lining, neutral greys etc. At the end of the day most tattoos require a variety of different techniques to produce solid, cohesive and beautiful pieces.
Proper tattooing technique usually starts with lining. Though there are many styles of tattooing that either do not require line work or use lining as accents only the average tattoo will start with an outline. As with any child's coloring book the outline of a tattoo acts as a guideline for the rest of the piece. Without a proper outline many pieces will age badly or lack contrast, in addition to this a poorly done outline sets the tone for the entire piece and will bring the quality down. Usually the biggest problem when outlining is either skill or the client. As an apprentice you will often lack the wrist strength and skill to draw a fully straight line, though over time and with practice this generally improves. Some of the more sadistic mentors may use methods like drawing with a barbell duct taped to a pen to help you build up wrist strength for tattooing.
Shading comes in a variety of forms and adding black and gray shading can help to add depth and contrast to pieces. Most shading is done through dilution of solid black and water to create a gradual fade by diluting the pigment load to parts of water. For example you could start with solid black, then finish with 1 drop of black to the rest water. There are several different ways to apply shading as well as different equipment. Most artists use different machines for shading and lining so that they can create the different effects. Generally shading machines run softer and create less damage to the skin so that pigment can be built up in layers without causing scarring, though it will also depend on the size and style of needle groups that the artist prefers to use.
Coloring is also a fairly hard skill to master, though unlike lining most artists will be quicker to learn the proper tattoo techniques for applying color. Applying color solidly requires both the right technique and patience as you are coloring pore by pore and trying to make the entire piece one even tone. In addition to this if you are adding more than one color you will need to learn the proper technique for smooth and even shading so that the color transition is not choppy. Smooth and even shading can be quite difficult and is something that many artists never master.