Kam, sorry, i can't find my pictures of this, must have deleted them
There's no hard and fast rules about
how to do this really. As ALEX** says, there's some reinforcement on the back of the liner which is a bit awkward. But the numbers of slits, size, spacing is really just down to your estimation. Don't make the slit too huge through or they might rub on the wheel.
I remember one of the torx screws was hard to reach too.
When you're trying to figure out which way to angle the slits, just think about the direction of rotation of the wheels when you're driving forward. With that in mind, angle the slits so that any crap flying off the wheel will deflect off the slit rather than go through the gap!