Is your slope really bad? If it is, then you need to level it somewhat before installation. If it's only a minor slope, then the installers should be able to work around it. The set does get installed on the ground (so NOT on top of mulch or anything else), and then you add mulch or pea stone after it's up. We also added railroad ties around the border to keep the mulch in place.
Tara
living a crazy life with 3 boys
I am thinking now
of grief, and of getting past it;
I feel my boots
trying to leave the ground,
I feel my heart
pumping hard. I want
to think again of dangerous and noble things.
I want to be light and frolicsome.
I want to be improbable beautiful and afraid of nothing,
as though I had wings.
~Mary Oliver