Advice for a newbie - Disintegrate...... kinda

Thanks for the link to the tutorial. That explains a lot. I could give that tutorial a D. Almost every step could be done more efficiently and there is absolutely no explanation of the theory or the reasons for doing what they did. Even though it has six million views (hard to believe that is accurate) it was pretty awful and obviously done by someone that does not know what they are doing.

If I saw your shot and the shot of the fridge you want to have the actor turn into I could be more precise, but here's how I would set up the production (filming) and the post-production workflow.

Production:

1. Set up a shot that is well lit, and make sure the background is uncluttered and not moving very much. No movement in the background is preferable.
2. Film a couple of takes of the actor snapping their fingers
3. Shoot a clean plate, which is a shot without the actor in the frame
4. Find a location that has your replacement element (the fridge) in the scene and make sure that the replacement element's lighting matches the lighting in the first scene
5. Film the fridge placing the camera at the same distance and at the same angle that was used to shoot the actor
6. To make the shot more interesting have another actor move into the scene and open the fridge and look in

Post Production:

1. Import the best take of the actor snapping their finger, the clean plate, and the fridge
2. Trim the shot of the actor from the first frame of the shot that will appear in the final edit until the frame where the actor starts to disintegrate but leave a few frames so the actor does not appear to freeze then fall apart using the footage panel
3. Create a new comp from the trimmed footage
4. Open up the comp settings and add enough time to the comp to allow the visual effect magic to happen and pay off the rest of the shot
5. Add both the clean plate and the fridge shot to the scene. It will help to pre-trim the fridge shot so everything you want is in the shot before you add it to the timeline
6. Arrange the comp so the clean plate is on the bottom layer, the actor is next and the fridge shot is on the top
7. Move the current-time indicator to the time when the finger snap happens and split the actor's layer (Shift + Ctrl/Cmnd + d)
8. You should be left with just a few frames of the end of the finger snap and the actor's arm continuing to move
9. Open up rotobrush or add a solid to the comp just above the layer of the snap and make a fairly loose cut out of the actor. Your mask does not need to be perfect because the actor is going to start falling apart as soon as you cut to this layer

Here comes the magic part - adding the effect

1. After you have a loose mask on the top copy of the actor layer apply Scaterize to the layer and keyframe the properties as needed to get the actor to fall apart the way you want
2. Pick a point in the timeline when the scattering of the particles is about where you want it to be for the transition to start to reverse and set a timeline marker
3. Slide the fridge layer so that it starts precisely at this marker
4. Mask the fridge layer so only the fridge is left in the scene
5. If you have another actor walk into the shot and open the fridge you may have to rotoscope the actor so they can appear to be in front of the background
6. Select the CC Scaterize effect in the Effects Control Panel and Copy the entire effect
7. Set the CTI (time) to the in point of the fridge layer and paste
8. Here comes the magic. With the fridge layer selected press U to reveal the keyframes that were added to the original Scatterize effect
9. Drag a selection around all of the keyframes using the selection tool or shift + select all keyframed properties in the timeline to make sure that all keyframes are selected

Now comes the expert technique. Right Click over the keyframes or go to the Layer Menu and select Keyframe Assistant>Time Reverse keyframes.

That is all there is to it. Your actor will break apart into little bits and the little bits will sturn into the little bits on the fridge layer and re-assemble themselves into the fridge. 

If you really want to get fancy create a bit of an overlap, set up an opacity fade and then pre-compose the layers that make up the transition and add time remapping to the pre-comp and add some keyframes and move things around to the particles slow down as they approach the transition and then speed back up as they assemble again into the new object.

Here's a screenshot of what the timeline would look like without the pre-comp:

There are several hundred different ways to use a click of the fingers to disintegrate something.  We would have to see your timeline with the modified properties of the layers you are using revealed (uu + PrintScreen + paste). It would also be very helpful to know what tutorial you are trying to follow. You have to be very careful about the tutorials you choose to use. Most of the tutorials you find on YouTube are by enthusiasts and a great number of them don't really know what they are doing so the tutorials promote inefficient, and in many cases, really awful dead-end approaches to creating visual effects. Make sure you vet your trainers and make sure they know what they are talking about. 

One of the best places to start learning After Effects is on the product home page under the Learn & Support link or right inside the After Effects Home screen. 

The process of changing a layer into something else (the fridge) involves at least two layers. If you want to use Scaterize you need to apply it to the original layer to break the object apart, and then apply it to the replacement image and animate in reverse or pre-compose and time remap, to make it assemble. Then you need to set up some kind of a transition to hide the cut. 

Show us a screenshot or point us to the tutorial and maybe we can offer an efficient solution.