Check out lesson 5.4 before reading this.
Summoning is a relatively simply concept but it is also dangerous and the reasons are described in the above suggest document.
There are multiple methods that I will discuss in the following, but be warned, not all of them will work for you.
Summoning a “Known” entity: It is relatively simple to summon an entity that has been known by other humans or by yourself for a long time. All it really requires is to link to them through looking at the energy that you know them by or something in mythology that was specifically linked them. And then simply “ask” for them to come, you might get bitch slapped if it’s a entity you have talked to before though.
Another method is literally throwing a link out to a specific kind of of entity, this is a terrible idea as you are literally opening up a open phone line and anything can latch onto it even if it's meant for a specific entity type. And then you don;t know exactly what that entity will be like.