Belief and single-mindedness are important factors in working miracles. This isn't so much that you need to "believe in love" because truth is a given...
"To believe in truth you do not have to do anything."
But if you DO believe in something of the ego (untruth, that is a problem. You need to be convinced of the opposite of what the ego teaches. Miracles cannot function when there is a spirit of doubt or fear.
"You can do anything I ask. I HAVE ASKED YOU TO PERFORM MIRACLES, and have made it clear that miracles are natural, corrective, healing and universal. There is NOTHING they cannot do, but they* cannot be performed in the spirit of doubt or fear.* When you are afraid of anything, you are acknowledging its power to hurt you."
This means that when fear and doubt are present, the mind is not in a state of wholeness enough for miracles to extend. Fear undermines your conviction that the truth is true, and puts you into a victim role. This means you have level confusion, believe in magic, and believe in death. Such a believe therefore supports the keeping of sickness and so your own will is now acting to try to thwart the miracle. And it will, because your will is free to do so.
To be single-minded means to not have fear or waver or doubt or fluctuate away from a sense of certainty, confidence and knowing, that the truth is true. Not having fear is a tall order for most people including most students of ACIM, which is why miracle don't come until LATER, after much ego has been undone to get rid of enough FEAR, and to heal enough of the SPLIT mind, that the mind can become single and whole-enough to facilitate a miracle.
You cannot "be in two minds" (a split ego mind) and perform a miracle, because this means you now have two wills. One of your wills will act against the other will, cancelling it out, producing a stalemate. This completely strips your will (shared with God) of its power to do anything, because every time one of your wills tries to will healing, the other one pops up and says "but, I don't think it will work", or "I'm still seeing sickness, it must be resistant to healing", or "I don't think it's working" or "I'm not doing it right" etc. Being in that kind of mindset is not miracle-mindedness. We need to be UNDIVIDED in our willing. Undivided in our willingness to will with God's will, which means having a single FOCUS on a single outcome, not two outcomes.
Here is Jesus speaking about the power of the mind:
"Few appreciate the real power of the mind, and no one remains fully aware of it all the time. However, if you hope to spare yourself from fear there are some things you must realize, and realize fully. The mind is very powerful, and never loses its creative force. It never sleeps. Every instant it is creating."
He goes on to tell us some essential ingredients for miracle working:
"It is hard to recognize that thought and belief combine into a power surge that can literally move mountains."
Belief must be present. "Miracles are by believers, for believers". If you do not believe enough, in the truth, fear will corrupt your efforts. There is a whole section in the course that talks about how the teacher of God gives a miracle and then might start to doubt that the healing has been fully given. Here:
"If the patient is healed, what remains to heal him from? And if the healing is certain, as we have already said it is, what is there to repeat? For a teacher of God to remain concerned about the result of healing is to limit the healing. It is now the teacher of God himself whose mind needs to be healed."
So this is another miracle-working tip. You have to be single-minded in belief such that you believe the healing is fully given and will be received without wavering or doubting, because if you go into doubt, "to remain concerned about the result of healing is to limit the healing".... you UNDERMINE the miracle.
Jesus tells us to give maximally, to trust the Holy Spirit can do HIS job, and to step aside and not go into fear:
"Whenever a teacher of God has tried to be a channel for healing he has succeeded. Should he be tempted to doubt this, he should not repeat his previous effort. That was already maximal, because the Holy Spirit so accepted it and so used it. Now the teacher of God has only one course to follow. He must use his reason to tell himself that he has given the problem to One Who cannot fail, and must recognize that his own uncertainty is not love but fear, and therefore hate."
This means not believing anything your eyes tell you, anything other doubting people say, even family members, even not listening to your senses, but instead TRUSTING and believing that the miracle has been given and it MUST produce results. To waver from this is fear, which is the cancel button. Healing should be given in the spirit of "I am so certain this will work that I will give this only once and I will not have to repeat myself."
Jesus goes on to tell us that we need to accept the atonement for ourselves SO THAT WE CAN GIVE HEALING TO OTHERS.
"It is in this that the teacher of God must trust. This is what is really meant by the statement that the one responsibility of the miracle worker is to accept the Atonement for himself. The teacher of God is a miracle worker because he gives the gifts HE HAS RECEIVED. Yet he must first accept them. He need do no more, nor is there more that he could do. By accepting healing he can give it. If he doubts this, let him remember Who gave the gift and Who received it. Thus is his doubt corrected."
Or to put it another way, he first accepts atonement for himself and receives love and healing from the Holy Spirit from God, and this heals HIM, and now he has the ABILITY to share that love and healing with others, because he is now willing to do so and will not tolerate anything else. Now he will offer miracles to others. And if he enters into doubt, it is because he has forgotten that the Holy Spirit was the one who healed him, and therefore the Holy Spirit will be the one to heal the other, THROUGH HIM.
Jesus warns us again, not to believe the ongoing APPEARANCE of sickness showing up, because it is a temporary illusion, and of the ego, and to reject it.
"One of the most difficult temptations to recognize is that to doubt a healing because of the appearance of continuing symptoms is a mistake in the form of lack of trust. As such it is an attack."
Therefore as you give healing, you must be willing to NOT LISTEN TO THE EGO in any way shape or form, otherwise you will become divided against yourself and the miracle will be cancelled. You must hold the faith and stay true to the truth for long enough to bring about the effects. That requires a kind of mental FOCUS and unwavering certainty.
If you are doubting what God can do through you, you are really doubting yourself:
"The real basis for doubt about the outcome of any problem that has been given to God's Teacher for resolution is always self-doubt. And that necessarily implies that trust has been placed in an illusory self, for only such a self can be doubted."
Trusting in yourself against God's will and trying to BE the one who heals will pretty much guarantee that it will not happen, because of yourself you are nothing. That is the unhealed healer. That is someone being magical, trying to enforce a healing from their own power, which is meagre. We need to learn to step aside and allow God to flow through us, to let the Holy Spirit be the one who knows HOW to heal, WHAT to heal, and to PERFORM the healing.
Jesus continued about the power of the mind, and also reflects that miracles are thoughts, by telling us:
"It appears at first glance that to believe such power about yourself is arrogant, but that is not the real reason you not believe it. You prefer to believe that your thoughts cannot exert real influence because you are actually afraid of them. There are no idle thoughts. All thinking produces form at some level."
So basically, your mind is very powerful and the mind you share with God is very powerful. It can exert REAL INFLUENCE over this world, over sickness and over death. You might be AFRAID to be aware of this fact, preferring to believe your thoughts don't really "do anything" or can't make anything happen. That's denial. And an ego defense against miracles. And it's a spirit of fear, which will ensure miracles fail. This is how miracles "go wrong".
So ingredient 1) have total belief and be convinced it is already done.
Ingredient 2) use the mind to "visualize" what it is that you wish to manifest as though it already exists.
Ingredient 3) do not go into doubt or fear or listen to the ego.
Ingredient 4) hold fast to the truth and be consistent.
Ingredient 5) step aside and allow Holy Spirit to be the healer. This will mean putting aside your own desire to heal, which is formed from a conviction that sin and sickness are real. Such 'fixing' is an ego reaction, not a true desire to help.
"I am here only to be truly helpful. (to heal)
I am here to represent Him Who sent me. (holy spirit)
I do not have to worry about what to say or what to do,
because He Who sent me will direct me. (and he WILL direct you specifically in words)
I am content to be wherever He wishes, knowing He goes there with me. (listening to guidance, trusting it, following it)
I will be healed as I let Him teach me to heal." (he will teach you to heal through your own healing/atonement which will then express and extend miracles to others).