The valve is abit of a bodge in that it will create a slight negative pressure inside the crank case, this negative pressure will suck shut the the turbo seals and stop them weeping oil (for a while)

You fit the valve in the elephant trunk->exhaust side cam cover breather pipe, the valve should allow to air to pass out from the cam cover to the elephants trunk, but not allow air to pass into the cam cover.

I got my valve from a scrap yard, just go and rip a bunch of them from cars brake servo's

Also here's a picture of a CA, showing which pipe the one way valve is put.