Evocation is the art of calling up a spirit to plain sight for the purposes of conversing with it and or charging the spirit with a task. There are many ways of accomplishing this, the most famous being that method detailed in The Lesser Key of Solomon the King, also called the Goetia.Techniques of this famed, and often, forboding art include the knowledge of the Lesser Banishing Ritual of the Pentagram(LBRP) as well as other methods of banishing/protection against any entity being called forth. The spirit, however, even in evocation, is not always commanded, but requested to do a task by charging it, for commanding it as Solomon did produces an angered demon or spirit, and your results may be the result of an angered spirit.(aka. you ask for money and your parents die in a horrible accident and leave you what is stated in thier will, which amounts to a measily $100 rather than what your older sibling got, everything else, or something around there) Now, after you request the spirit's presence, using your energy/chi/aura/ empower his sigil and ask him to perform the task you wish him to do. Once you both come to an aggreement, give him the license to depart and do the banishing techniques to rid you of any left negative energy. Now, an important part of the ritual is to forget about it, and results will come eventually.(it is good to give the spirit a deadline to be sure your ritual was a success.) Also, after the ritual, you should record what you did for future referrence.
Evocation vs. Invocation
Many of us confuse invocation with evocation although there is a very important difference. invocation is mostly done with benign spirits. Such as invocating the god and goddess in a wiccan ceremony. however evocation is mostly done with lower and less friendly spirits. evocation should never be done with gods or higher spirits, because it is considered an insult.
With invocation the spirit in question is invocated into your circle or temple. no protective deviser are used against it, and it is done by requesting and not forcefully.
With evocation however the spirit is summoned by force, using various god names and magical words. The spirit is then forced to appear inside a device specially made for this purpose, mostly a round black mirror mounted in a triangle with various god names written on the sides of the triangle. This evokes the spirit inside the round black mirror and the triangle imprisons the spirit within it. Then you can ask the spirit to do your bidding.
So where with invocation the spirit is asked freely to come, and can do as it pleases. With evocation however the spirit is summoned forcefully and contained within a special devise. It should be obvious that higher spirits shouldn't be evoked, since it will prove hard to contain them and they will not be pleased by it.