How do I set a slab that is up?

I play Minecraft on my X-Box 360 and want to build cars you have to have the slab on top of the block, that will not work for me, as far as I've tried!

This is possible on the PC edition, but the Xbox 360 is already older. Originally, this feature should come in an update. To put it briefly: it does not work.

Ohh…