This means that you cancel, as in undo, your belief that an illusion is a reality. Whatever has been asserted by the ego as true or real, you cancel it. You confirm it is false and delete it from your mind.
So if someone seems to be sick, initially you probably believe that the sickness is real in various ways. You've seen it, heard about the symptoms, know the story, believe in the body, think it's actually happening, give it value, attribute it to various fake causes, etc. All of that needs to be collapsed because it is not God's truth.
So to cancel it, you first need to forgive, i.e. undo the delusions. You need to come to recognize that all of it is an illusion, NOT a reality. There is NOT a real sickness. The person is NOT a victim. The symptoms are NOT real. It is NOT really happening. The story is NOT the proof of how it happened. The explanation for it is NOT the real cause.
Cancel out all those false beliefs. Just reverse them. If they seem true, and if it's not God's truth, it is FALSE. This undoes all of the belief in the reality of the problem in your mind so that you're not SUSTAINING the sickness. Believing the sickness is real, at all, empowers it, fuels it, feeds it, keeps it, protects it, desires it, and chooses it.
So negate, cancel, undo, collapse, disappear, dissolve, make vanish, all of everything about 'it'. Get every single belief about the sickness being real out of yourself and out of your life. Get rid of all beliefs in sickness in yourself and others, let there be no permission for sickness to stay, reject all suggestions by everyone that sickness is real or true or permanent or justified, and do not believe in it.
Cancel the lies in whatever way that seems to be needed to get your mind to a place where you find yourself starting to state the core truths... like, there isn't even a sickness there, the sickness does not exist, there is no sickness, IT IS ALREADY GONE.
As you approach the truth, you are empowering yourself, by undoing those illusions. And you may (should) start to experience a building up of POWER in you. That's the Holy Spirit. Because you're aligning yourself with God's truth, you should start to feel His flow of 'electricity'. Your words are becoming more powerful, they are starting to contain Spirit, and you're expressing a higher vibration of love.
This is kind of like a prayer that ascends up to the truth of the Atonement: there is no real sin, nothing has happened, the sickness cannot exist because God did not create it. Or even, recognizing the sickness was a defense against the Atonement, there is no real NEED for the sickness, therefore the need for it was the problem that caused it, the need is unfounded, therefore the symptoms are unnecessary defenses. Illusions recognized as illusions must disappear.
As you hook yourself up to the truth, plugging your mind into God on one end, out of the other end you can utter miraculous healing words - commands of what MUST be, given God's truth. Dispel illusions, replace them with a higher order, something more symbolic of divine health. Speak LIFE into people. Stop them from constantly reinforcing, repeating, reiterating and compounding the reality of their problems. Someone needs to stop the madness and step in and make a statement of THE REAL TRUTH.
Later, you will then find that because you undid your own illusions about the sickness being real, you will not need to make much effort to defend against ego assertions that the healing didn't work etc... those thoughts mostly will not arise, and if they do you won't believe them. Because you already got done undoing the belief in those truths. This puts you on a much firmer footing for being convinced that, believing, that the healing has been accomplished by God and that the person MUST be healed.
Cancelling the delusions about sickness being real automatically cancels out the foundation for doubt, and doubt defends against miracles.