Stone slabs are the half heights version of their respective blocks.
This guide will briefly discuss the method of making the stone slab. Firstly, you need to bring some pieces of cobblestone. Make your way to a statue of a horse statue and start breaking the step like structure with your hammer.
Head to the furnace and use the cobblestone pieces on it. Take this stone into your inventory.
Once you have this stone in your inventory, you will be able to create a stone slab. Move to the crafting table just next to the furnace. Just use three stones on the crafting grid to make stone slab.