Evokation is calling an entity to you. unlike Invokation, drawing an entity to you, the being you evoke has a choice on whether or not to come. If it comes from being called, you can draw much more energy from them then if you bind them and force them to come. Normally, evoking an entity involves the knowledge of their name and a means of preventing them from invading your conciousness.
First Step is finding the name. This isn't very hard. A simple method would be just imagine the kind of being that you want. If you concentrate, the name will normally come to you.
Next you have to set up mental barriers. You can imagine a wall around your conciousness, visualize this, feel the walls, and generally your mind will channel energy into a barrier around itself. Depending on what type of being you are evoking, you may need to add extra barriers or even use a pentacle.
Next is the actual calling. You will find a ton of incantations, chants and spells for this, but the simplest is just calling the name, mentally, three times. Since most entities that can be invoked will not exist in the material, the time it takes for them to travel to you should be small, two minutes at most.You will know if the being chose to come because you will feel their presence.
Its hard to describe what a presence feels like. You might feel like someone's watching you, or as if there is something always behind you. The most obvious indicatior is if they attempt to break into your mind. You will feel a panic, as well as a force pressing at your skull. Concentrate on a single thing if you can, or try to push the being out. If it doesn't let up, banish the entity at once. If you can, however, placate it and attempt to commune with it.
Always try to study an entity before calling it. If you cannot find any information, give up and try to find a different entity. Always meditate before calling.