1. Start out by creating a blank Google Slides presentation. (use the Blank slide layout)
2. Start designing the first slide.
3. Duplicate your first slide.
4. Move something on the second slide to start the animation.
5. Duplicate your second slide.
6. Repeat this process over and over again until your animation is complete.
Resources
EASY stop-motion animation with Google Slides by Matt Miller
A 28 minute tutorial video
This really is awesome
Use the Creator studio add on to turn your Google Slides into a movie file that you can download.