Birch Slabs are half-versions of their respective blocks. And it can occupy either the top half or the bottom half of a block, or both.
To make Birch Slab, you will need Birch Wood Log and Birch Wood Planks. Please follow the following steps to make Birch Slab.
Birch Wood Log
In order to make Birch Slab, the basic item you will need is Birch wood Log. Find a Birch tree and use your iron axe to chop down a piece of Birch log. You can find Birch trees in the Birch Forest Biome.
Birch Wood Planks
After that you will need to use the crafting table to craft birch wood planks. Go towards the crafting table and open up 3*3 crafting grid. Add birch wood log in the center or any cell of the grid and you will get four birch wood planks in the right-side box of the crafting table. Add these planks into your inventory.
Now you will be able to craft a Birch Slab using the crafting table. Open up 3*3 crafting grid and add three Birch planks in any row of the crafting grid as shown in the image below. You will get six Birch slabs in the right-side box of the crafting table. Add these slabs in your “ready to use” stock.