Traditional evocation is a technique magicians use to work with entities. It differs from invocation in that the magician is summoning the entity into the environment around him/her. Evocation is typically used to summon an entity so that it can perform a specific task that the magician requests of it. Some forms of evocation involve summoning an entity and then compelling it to aid you by invoking other entities that are opposite of it. This essentially amounts to threatening or compelling the entity into doing the task for you. I recommend against it, and would suggest bargaining with entity, and honoring your end of the bargain. When doing evocation, it's a good idea to consider why you are doing the evocation, and if the task you want the entity to perform can be accomplished another way. There will be certain types of tasks that are better suited to entities and certain ones that you can resolve yourself. I use evocation when I want the entity to perform that involves influencing a specific behavior or activity in my favor, but still leaves me with the actual decision. For example, one entity I work with is focused on making me aware of potential opportunities. I still have to choose to follow through on those opportunities, but the entity takes care of an essential task that I simply couldn't do. Its focus is on scanning space/time for the opportunities and than making me aware of them. It's not a skill I have, but it's a skill that entity has.
On the other hand, evoking an entity to do something you could handle yourself is wasteful and also keeps you from sharpening your own skills. It's important to recognize this distinction so that you can improve your skills. For example, I once wanted to buy a car. Instead of creating an entity, I did an enchantment to obtain the money I needed to purchase the card. I already had a job, but I knew I needed more money than I'd have available at short notice. By doing the enchantment, I was able to focus on very specific details the entity might not have considered. I realized I needed to attend to the task, instead of assigning it to an entity. It's a case of picking the right tool for the right job, and with evocation, it's better to have an entity do something you can't do, then have it do something you could've easily handled yourself.
In ceremonial magic, evocation is done with specific tools, incenses, and other props that are used to create a specific atmosphere that accommodates the entity as well as the altered state of mind the practitioner needs to be in to work with the entity. However, evocation can also be done with collages , paintings, and other techniques that are non-traditional but still enable the magician to make a connection with the entity. Although some magicians will claim that traditional approaches are more powerful, I've found that using more artistic approaches to evocation has been successful. I think it comes down to understanding that evocation, much like any other magical technique, is really based on your understanding of the fundamental principles at work. If you understand those principles, you can make evocation regardless of what tools you use.
Evocation isn't limited to evoking entities. The magician can evoke specific behaviors or emotions, or even do an evocation of another person or a past/future version of him/herself. Just as with invocation, it ultimately comes down to connection, and understanding how to use evocation to create and embody a connection you want to manifest in the environment around you. I have successfully evoked people into my life, by using specific attributes as a way to create a connection with people who had those attributes. Evocation operates on the principle of connection and names. The name of an entity, or an attribute, or a person is a powerful tool that allows the magician to create a connection to the manifestation of the entity, attribute, or person. That connection is used to pull the entity or person into his/her life and to embody the attribute from within him/herself in the external environment.