The definition of a Familiar Spirit has changed and been moulded with overtime. With many New-Agers and Wiccans, a familiar is an animal (usually a cat) that embodies a spirit, and serves as a guardian for the fellow witch. However, the old (and true) definition of a Familiar spirit is much, much different and complex.
A Familiar is actually a spiritual servant is created for the witch, and is created from the witch's psyche instead of resting in a physical, sentient being. Familiars are just as strong as demons and psi-ball spirits, but creating them can be a challenge. Here is a guide on how you can create one, and how you can have it (he/she/them for some people) be your servant.
The first thing you want to do is think of a name for your spirit. No, not names like Samantha or Brown Jenkin. You can use these, but it's better to name the spirit after the intentions it's put to use on, like Lovebringer if you wish for a partner, or a name with a translation of your intention, such as Aadya for strength and willpower. Simply naming your spirit Strength or Willpower is also a viable option. The possibilities are endless.
Your next step is to bind the spirit. The easiest way is to make a sigil, which can be done in a variety of ways: from writing a symbol on a piece of paper to creating sigil wheels.
You then create or find a physical object which the spirit is bound to when it is not doing work. This keeps it from leaving or sneaking away. This is one of the most enjoyable parts of creating the spirit, so get as creative as you can/like. I usually mark a symbol or the familiar's name to the object.
After you do this, you are going to imagine what your spirit looks like, even if it may look slightly different on the astral plane. I sometimes draw my familiars concept and hang it on my altar for later ideas. This is the "incubation" period, but you have yet to summon it to you. That is the next step.
Design a ritual for summoning the Familiar. I usually do so on an altar containing the Familiar's sigil, bound physical object, and image. I sometimes put objects that will be relating to the familiar's assigned duties, e.g some medical herbs for a healing familiar. You can also give offerings, such as florals, food...the possibilities are endless. When you begin the summoning, trace its sigil in the air with your finger, a wand, or a dagger while you say the Familiar's names 3 or 7 times (these two numbers are lucky and magical). Keep thinking of the Familiar's name while you look at its image on the altar. If you lose focus, repeat the sigil tracing and name pronunciation while you focus on the image.
repeat this ritual everyday until you feel the Familiar come into being. These rituals help minimize influences from errors that may come up from doing something for a first time, so reflecting and re-working on your rituals as you go along helps you create a Familiar spirit to your liking (be wary that there will be imperfections, but nothing can be completely pure and free of flaws). These rituals play a huge role in the incubation period of your Familiar.
After your Familiar has been "born" (a truly wonderful thing, indeed), you can start commanding your Familiar spirit for tasks. You want to create a bold proclamation at the time of the summoning. It can be short, about 8 or so sentences long, and should be clear, concise, and to-the-point. In your proclamation, name the spirit, say the purpose for its creation, and the consequence if it shall disobey. Remain calm, and show no fear. You can point a dagger at the sigil for emphasis while you do the summoning.
You need to create a "return back" ritual to use once the Familiar has either finished the task or if you feel it has done enough to your liking. If you send it out for long tasks and journeys (such as pursuing an individual for hexes or finding a lost love), be sure to call it back once a week (or more often depending on how desperate you are for the familiar to complete the task) for a short rest period.
This familiar can be with you your whole life or destroyed right away, depending on if you feel it is doing your bidding/tasks. More than one familiar can be kept by one witch, but be very careful. Never lose their sigils or the physical objects they are bound to, or the Familiars may destroy themselves. I recommend, if you want to keep your Familiar(s), to keep their sigils on an altar. Over a period of time, Familiars can develop their abilities and become better servants, so I also recommend being patient and attentive.
I hope this was helpful...or at the very least beneficial. I tried to write this without it being messy.