To perform a miracle, you must attune your belief and awareness to the state of truth that you are bringing about in the future.
With your mind imagining the future, picture the end result of healing in the state of it having actually happened. This is in effect accepting the atonement, that nothing unreal happened and only God's truth has happened already.
To do this goes against the ego because the ego holds onto what it believes is true now and tries to reinforce it, pulling up the past to prove and keep it.
"Overlooking" (forgiving) current appearances is a key part of miracles. You have to overlook how things seem now, recognizing that the current condition is the result of the past and you can change your mind to produce a new future completely unlike how it is now.
So in a way, overlooking the current state is like looking to the future.
Then, trusting that you have conceived of what the result WILL be like, regardless of how the world or body appears now, you remain fixated on that truth and believe it.
You believe it in the sense that it is "already done". So you bring that future state of "after the healing" back into the present. Believe now that you are already at that future moment, after the healing is already accomplished, putting the obstacles to healing behind you.
Having FAITH that the unseen will manifest as truth even if it is not showing up yet, is a key to miracles. "Miracles are taken first on faith."