When I did this on a route a while back I created the area at the top and bottom of my hill, making sure to leave area for where the slope would be. It takes a bit, but run a bunch of single-spline tracks from top to bottom over the slope area (make sure none of them connect by holding shift, or you can go back later and disconnect the splines). I mean a bunch of tracks, making sure you cover at least every two ground blocks in the rows (this may mean if your hill is rounded you have splines a lot more bunched together at the top than the bottom). Once your tracks are laid down with splines separated; and the spline points are set height (either before hand using Surveyor options or afterwards by clicking on each point with "Adjust height") then go through and click on each track with the Adjust Ground to Spline (whatever the actual name is)... Even if some of these "undo" what was done by a previous track it adds to the slope's authenticity.
I did this with a couple areas on a route and they ended up looking pretty good - both with long shallow slopes and short steep slopes, and in between. Definitely helps break up the steep cliffs and seems a ton easier than trying to use the smallest cursor and making little changes between each "step" and no steps.
Honestly I may have gotten this from here or somewhere else (maybe one of those youtube vids already posted, I didn't look at them), I don't remember. I could make some screenshots if someone wants.