Add an undo/redo method to core graphic draw app

I am trying to implement an undo/redo method using NSUndoManager. I have asked other questions on this, but am still stuck.

Where I am at the moment is as follows:

.h
 NSUndoManager *undoManager;
 @property(nonatomic,retain) NSUndoManager *undoManager;
 
 .m
 @synthesize undoManager;
 

[undoManager setLevelsOfUndo:99];
viewdidload:

NSNotificationCenter *dnc = [NSNotificationCenter defaultCenter];      
 [dnc addObserver:self selector:@selector(undoButtonTapped) name:@"undo" object:nil];   
 [dnc addObserver:self selector:@selector(redoButtonTapped) name:@"redo" object:nil];
 
 - (void)resetTheImage:(UIImage*)image
 {
     NSLog(@"%s", __FUNCTION__);
 
    // image = savedImage.image;
     if (image != drawImage.image)
     {
         [[undoManager prepareWithInvocationTarget:self] resetTheImage];
         image = drawImage.image ;        
     } else {
         NSLog(@"That didn't work");
     }
 }
 
 - (void)undoButtonTapped {
     NSLog(@"%s", __FUNCTION__);
     [undoManager undo];
 }
 

I get "That didn't work"...

I would appreciate help. I will post the answer to my original question when I figure out what I'm doing wrong.

---EDIT---

I have changed resetTheImage as follows:

- (void)resetTheImage:(UIImage*)image
 {
     NSLog(@"%s", __FUNCTION__);
 
     image = savedImage.image;
     if (image != drawImage.image)
     {
         drawImage.image = image;
         savedImage.image = image;
         NSLog(@"undo image");
         [[self.undoManager prepareWithInvocationTarget:drawImage.image] image];        
 
     } else {
         NSLog(@"same image");
         savedImage.image = image;
     }
 }
 

However, confusion rains - it may help if someone (Brad?, Justin?) can provide a bullet point list of the steps I need to take to get this working. For example:

. Add notifications in ...
. Trigger notifications....
. Have undo /redo buttons point to...
. What methods/functions I really need to build..
....

I wish SO would allow me to give you more points than 1..

(It doesn't help that my "o" key is getting wonky)
It does help that I had a baby granddaughter yesterday :))))

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