Question: mcreator how to make slabs?
Answer: Extract the . jar file for Minecraft 1.12. ... 
Go to the files.
You will find a model folder. ( NOT THE MODELS FOLDER IN THE TEXTURE FOLDER)
Use search to find slab.json in MCreator. ( cause others cannot have added textures)
Use the model with textures in MCreator.
Launch to check it if it works. ... 
Enjoy or not!