There are many methods of invoking any demon. A few aspects to go through here:
Before invoking it is always advisable to learn how to banish. You do not want to summon something if you do not know how to banish, you immediately lose your " power " here. That's how bad scenarios take place, because people lack information. There are many methods of evoking/invoking as well.
But prior to all of this, invoking and making contracts etc.. you should study your demon in question. Every demon has different characteristics, energy imprints, etc. just like a normal person. You should study, hear stories and then actually try and starting to work with them, if desired.
When we speak of contracts, and it's mentioned here " ... is also a contract that once said cannot be changed ".
When you create a contract it is because it's beneficial to both parties, not only one, More often than not demons do not, like in Hollywood movies, want your soul or any of the likes. It is a lie that you can sell your soul.
A contract is simply an agreement that has a desired purpose, where two parties will play a part on. This meaning, you will both work together to achieve a goal, when that goal is reached there is a price.
This price can be many things, this is why when/if you do a contract, you first talk about it until you both get to an agreement. Just like you would in the physical world. This is not something to be taken lightly, it should be taken seriously and it should not be broken.
It's not something I, personally, would recommend to beginners. If this is your goal, then take your time and work your way up to it. Study, read, listen and study some more. You can never have too much information.
It's also important to note the difference between invoking and invoking a demon. Invocation such as what is used here, involves welcoming a demon (in this case) to assist you. In the case of an invocation, the demon is not bound by your will and does not have to do anything that you say, the demon may also chose not to show up or bother with you. Invocation tends to be used by those more experienced in dealing with spirits as it requires an open mind, and that you treat what you are dealing with, with respect.
Evocation on the other hand is forceful, when done correctly, an entity will be bound to some object or will and will be forced to show up. Evocation relies on the imprisonment of an entity to strike a deal. It is important to note that an evocation must be done correctly lest you risk nothing happening at all, or worse, you will offend an entity at your attempts to imprison them, and will be left dealing with the aftermath.
Many people, such as those practicing forms of theistic Satanism and Daemonolatry find the process of evocation to be disrespectful as in many cases the practice involves respecting and honoring the demon. The process of evocation, while viewed as safer by some, has much more severe effects if done incorrectly. Hence why many chose to invoke. Although there is far less magickal protection in an evocation, the practitioners defense comes from their neutrality and respect towards the entity which they chose to work with.
Overall I would suggest refraining from any acts of summoning demons until you have a greater understanding of them, and do not see them as an inherently evil force (although they may be at times it is up to you to find demons willing to work with you). It is important to know which demon you plan on working with, and know what to expect that you must do in return. Summoning is not something to be thrown around, and requires much practise and skill.