So you want to build a servitor. While there are numerous methods for building a servitor, constructing your first servitor requires a lot of forethought on your part, and at least a passing familiarity with sigilization techniques.

First, a bit of theory: A servitor has the same essential nature as any other thoughtform. Its nature is "visible" in the astral, like any other godform, demon, or spirit. It rules a specific sphere of psychic/emotional energy. It draws all of its initial energy from the mage (or mages) who create it, and usually sustains itself from those same mages (except in certain circumstances.)

When a servitor's task is finished, it is usually reabsorbed by the mage (or mages) so that it's psychic/emotional energy is not wasted and can be redirected by the mage at a later time.
 

This use of energy is rooted in the concept of a spiritual plane that rides coterminous with our material plane. In fact, all magickal acts occur primarily on this spiritual plane. While a great deal of effort by various magickal groups has gone towards the naming and mapping of this spiritual plane, it is only necessary for this ritual's purposes to simply acknowledge the existence of this level of reality.
 

In this level of reality, thoughts themselves have a form and function. Any thoughtform which arises from more than one person can have an effect on all people as a whole. Perhaps it is most efficient to think of this as the collective unconscious, as described by Carl Jung. Thoughtforms that arise from more than one person would then be seen to be as greater and lesser archetypes.

A thoughtform which is deliberately placed into this environment by a mage has the benefit of advanced knowledge of its environment. This appears to be why a solitary mage can create a thoughtform which can go toe to toe with ancient archetypes and godforms. For most archetypes and godforms, a kind of Lamarkian evolution has to occur, which can take several cultures to fully evolve. (This is the central focus of comparative mythology.) But for a servitor, an extremely focused task is given, with a reservoir of energy to pull from, and the landscape is already pre-mapped.

This acts to further explain the attraction of servitor spirits over those pre-existing thoughtforms in the collective unconscious which have their own set rules to which the mage must conform for their evokation.

Second, an overview of servitor types. As stated previously, a servitor is birthed by a mage and sustained by the mage's psychic/emotional energy, unless there are extenuating ritual circumstances.
It is easy to simply assume that what a person does in a magical context for hir own benefit does not spread out into the lives of others, but that is simply not the case. I, for one, happen to have a fairly strong background in "wiccan theory" as regards the karmic cycle, and, because of this, I tend to focus on "good" servitors when I construct them. I don't expect everyone out there to do so, and I think we can all agree that somebody at some point has made themselves a servitor devoted to lust.

Lets use this as an example.
1. You want to get laid. In fact, you want to get laid a whole lot.
2. So you spawn off your lust into an entity to go out an hook you up.

What's the problem here?

Spawning off your lust thereby reduces the desire for sex, but now you got a spiritual entity trying to get you laid. So, when it does happen, you don't enjoy it. In fact, you become a little antagonistic about it, and your antagonism develops into destructive behavior. (sex w/out caring about sex... talk about masturbation)

This is a prime example of creating a stupid servitor... you should have stuck with a sigil, or a candle spell or something simple. instead, you've just infiltrated the ideosphere with another useless and weak servitor, and you've lost a bit of what makes you human.

As for what the servitor lives off of when it ain't feeding off you, or you ain't feeding it, this is another area which should be addressed. I happen to like hijacking commercialization, or media events, to use as servitors. (movies make a good time-frame servitor energy force)

Say you want to do something like challenge conservatism on your school campus with a servitor. (this is an old example, new movies are coming out all the time, use them) you link your servitor to the movie Election, and define it as feeding off the emotional energy generated by movie go-ers who see this movie in the theatre. During the theatrical run, the servitor is up and running, then, after the movie leaves the theatres, it comes back and splices itself back into your psyche.

Other mages (more astrologically inclined than myself) can link a servitor to a specific planet, and, while that planet is up influencing us, the servitor is online. This seems like a good, all round timeframe, especially if the 'sphere' in which the servitor functions is connected to the 'sphere' of influence the planet represents.

As for mutation, servitors generally mutate into the most powerful and potentially useful form to fulfill their statement of intent, which is why a clearly defined (try e-prime) statement is crucial. When this occurs, sometimes you get lucky, and they don't go untied, but its better to be clear in the beginning, rather than to be surprised later on.
 

Finally, let's look at the process of creation.

There are as many number of stages as you feel so inclined to label, but for simplicity's sake let's stick to an eight-step system.

1: Define sphere of influence

What general area does this servitor fall under? What archetypal image might be connected to the servitor's task?

2: Define tasks

What is the servitor supposed to do? Where is it headed in the ideosphere?

3: Define duration

How long does the servitor need to be operable? Do you plan to keep it dormant until needed? What's the feeding schedule?

4: Define task symbolism

What symbols can you incorporate into the creation of the servitor that would correspond with the tasks at hand?

5: Define servitor name

Use whatever sigilization techniques you feel comfortable with in designing the servitor's name and glyph.

6: Define material base

Do you want the servitor connected with a material object beyond the glyph? Does the servitor need a physical shell for you to more fully visualize it fulfilling its function?

7: Define astral shape

What form does the servitor take on the astral? (subject to possible mutation) Begin with visualizing the glyph, and follow it through until you percieve the underlying spiritual force it represents.

8: Detonate gnosis for birthing process

Continue with the visualization, create a peak meditation experience, and fire the energy into the glyph/astral form.

Hopefully this has helped further your knowledge of servitor-based magick.