By definition, invoke means “to call forth or upon [a spirit] by incantation,” (source). The act of invoking a spirit is the process of calling it to take vessel in your body. It is a type of “Horsing”. A way to look at it is that you are allowing a spirit to take hold of them. Horsing means, at least according to the dictionary, “something on which a person rides, sits, or exercises, as if astride the back of such an animal,” (source). A famous example is the ritual of “Drawing down the Moon”. In which, the priestess invokes the representation of the moon (typically it is a goddess of the moon, but sometimes it is a spirit too) and becomes the moon itself. She basically takes it inside of herself and becomes it.
The purposes of such a ritual can vary from it being purely a religious ceremonial act or a ritual meant to empower the practitioner’s power in the hopes of gaining their desire. A ritual of invoking the spirit usually, but not always, include such steps as cleansing of self, cleansing of space, calling of the spirit, invoking the spirit into your self, performing the task when the spirit is inside you, dismissing the spirit from yourself, closing of the ritual, cleansing of space and self.
Some examples of rituals involving invocations of spirits/deities
I have yet to truly invoke a spirit, but I certainly have many thoughts and ideas when it comes to the ritual layout itself. The idea of using masks for ritual work and magic fascinates me so much! So, its no surprise that I want to use masks when I want to invoke a certain spirit. It can be used as a tool to help you connect with the spirit even more. You look more like it, and thus you become more like it. It certainly would make a more interesting ritual too.
anyways that is enough of my rambles…