The first step is to decide on a name for the familiar spirit. You can use any name you like, but a name based on the familiar spirit's purpose seems to work best. Some examples of this type of name might be Moneybringer, Guardian, or Healall.
Then, use the name to come up with a sigil that binds the familiar spirit. My own preference is to use a planetary kamea or a sigil wheel to make sigils, but any method of producing a sigil will work.
The next step is to create some sort of physical object with which to contain the familiar spirit when it is not out doing work. This object will also serve as the physical receptacle of libations. You can get as simple or creative as you'd like. I just take a small disk of wood, carve the sigil and name onto the disk, paint it, and generally try to make it look "magical."
The next step is to decide what you want your familiar spirit to "look" like, even though it will only exist on the astral. I just draw something simple to aid my visualization during the "incubation" period. If you're a fairly good artist, you could even take your drawing, put it in a frame, and use that to contain the familiar spirit instead of a wooden disk. If you decide to do this, write the name underneath the drawing and draw the sigil on the back. You won't be able to see the sigil, so make another copy to use when you summon your familiar spirit.
Next, design a ritual for creating the familiar spirit within the disk or depiction of it. Any opening ceremony that you use for spell work or summoning probably can be used this, too. When you are ready to begin creating the familiar spirit, trace its sigil in the air in front of you while pronouncing its name three times. Visualize the appearance of the entity or stare at its deptiction if you prefer. Continue with the visualization for about fifteen minutes. You may also want to pour on it libations of milk, egg whites, or blood. If you begin to lose focus during visualization, repeat the sigil tracing and name pronounciation part of the ritual and keep going.
Repeat this procedure daily until you can feel that the entity is becoming more and more real on the astral level. The initial feeding and preparation is called an incubation period. Incubating your familiar spirit gives you more of a window for error and removes a lot of pressure to do everything exactly right on the first try. To start with, allow a week to incubate your familiar spirit before giving it a task.
If you banish before magical work, make sure to remove the disk or depiction from the room before banishing during the incubation period. You will banish the developing familiar spirit. After sending it out on a task, however, using a banishing ceremony can act to give it a little push in case it wants to hang around instead of doing work.
After the incubation period, you can command the familiar spirit to do a task for you. Below is a sample of what you might say the time of summoning. This charge to the spirit has not been handed down by King Solomon, Merlin, or anyone like that, so feel free to revise the wording if you need to. You may want to point a ritual dagger or wand at the disk or depiction for emphasis:
Hear me now _____________! I, who created you from the astral substance, charge you to rise from the disk/depiction bearing your sigil and name. I, who created you on the astral plane for the sole purpose of working my will upon this plane, command you now to hear and fulfill this task. I, who feed and fuel your very existence, command you to ________________. Fulfilling this and all tasks I give you is your duty and sole justification for having been created. Obey now with full cooperation and faithfully serve your lord/lady who created you and who will destroy you if you are disobedient.The task should be as simply and as clearly stated as possible - ideally a short, declarative sentence. This charge is more heavy-handed than what I would recommend using for an independent spirit that you didn't create yourself. However, this spirit is yours and was created to do your will. It's also very simple-minded, so the strong, direct approach is needed.
Come up with a "calling back" ritual to use once the task is done. That's normally when you should feed your familiar spirit. If you send it out on long tasks such as protection of another person or menacing, be sure to call it back at least once a week to feed it. If your familiar spirit fails consistently to complete tasks, make good on the threat to destroy it by burning the disk or depiction and thoroughly banishing it.
If you want to know more about creating familiar spirits, the two best books available are Creating Magickal Entities: A Complete Guide to Entity Creation by David Michael Cunningham, Taylor Ellwood, and T. Amanda R. Wagener and Familiar Spirits: A Practical Guide for Witches & Magicians by Donald Tyson. Each book has ideas and methods not contained in the other, so if you're really interested in this area of magic, you will want to get both books.