Ill check these links out, I'll probably need this knowledge cause I want to start conjuring. Any advice for working with dark entities like Demons, Slenderman, etc.

Don’t start with conjuration itself if you want to work with demons or negative energy thoughtform (slender man). Learn to master the self first.
An unfocused mind that is not properly trained will result in you calling forth unwanted thibgs. When doing evocation you will conjure what you focus on which might not exactly be what you desire.
For example if you want a loving succubus and you write a letter to Lilith with fear in your heart and not love you will call forth an entity of fear and not love. It will make you fear.