Really there is no solid answer to this question, there are many ways one can be called to work with a particular demon and many reasons one might seek council from a Demon or Dark Lord. We all have our reasons and sometimes it is the most random events that call you to invite demons to join you in your life.
It might be you open up a book, read about a demon and just feel a pull towards that demon. This is very common, each demons name and sigil carries in them that demons essence, and when we are pulled to that essence there is nothing wrong with seeing where it goes. As long as demons are approached with respect and appreciation for who and what they are.
You could also see or hear that demons name in your daily life and just feel a pull to work with them. Sometimes they will reach out to us to get out attention so you might hear their name or something happens that makes you think of them, then you continue to think about them. This can be a calling and there is nothing wrong with following it and exploring it.
You might have a dream, or an vision where this demon approaches you. This can be powerful and life changing for some people. Demonic energy is incredible and when it crosses our path it can start and incredible journey for transformation. Not all dreams are demons reaching out to us, and if you are nervous or not ready, you never have to feel forced to follow up on this connection, but if you feel pulled and have the desire you should explore it.
Or you might just feel curious and want to learn what a particular demon has to teach. Nothing wrong with that, the Dark Lords and Demons are incredible beings! Working with them can bring incredible changes and transformations to our lives and many people have found positive life changing journeys from walking with demons.
Really in the end, if you feel pulled towards a particular demon, if you feel comfortable, you should explore it.
We always recommend starting by reading about that demon, and learning about that demon. What are their likes, and dislikes? What is the best way to approach them? It never hurts to be prepared and to come up with a plan to approach and work with that demons. Some demons like a more quiet setting and smaller rituals, others are all about ceremony and grand rituals. Each demon is different and it is important to research them and com up with a plan for approaching them in a way that is respectful, and also honours them.
Being called to work with a demon can be a very incredible and life changing experience. When you feel called it is nothing to be scared of, it can in fact be and incredible time in ones life and the start of an incredible spiritual journey to craft the life of ones dreams!