it’s kind of a weird process i’ve developed.
I start two documents that are the size i want.
One has each item in a different layer, i can move the items around on the canvas with the mouse. In this case, since the car needed to go into the can, I made the inside/top of the can a layer that was below the car, and the front of the can was above the car (in layer order). Then i just use the transform and move command on the image of the car. (while the car is shrinking, both the inside AND front of the can are on a layer beneath the car)
Each time i adjust what’s on this canvas a bit, I perform a COPY ALL on it and paste it into the second document. So i just move,copy,paste,repeat. Then when you animate in photoshop (i have photoshop elements, so i use ctrl+alt+shift+s, then select GIF and choose animate and loop. If you have regular PS, you might need to use adobe imageready to do this…), each layer is a frame of the animation. You can select the rate at which the layers display. (however, in photshop elements, you cannot choose time for each specific frame, so if you want one frame to display longer, just repeat the image in that layer multiple times)