A good way to get a flat ground is trying to understand what the hoe basically does and how it works.
In general it's like
if you stay on higher ground, it raises the marked area to the level you're on and vice versa.
If you want some sort of transition, locate yourself directly on the point you want to adjust and use the hoe right beneath you.

Keep in mind that the effect is also affected by the surrounding materials.
For example if you try to use the hoe on a block (area) that contains both soil and stone, it might have a bigger effect on the soil area than on the stone one.

Another thing you need to keep an eye on is the height difference between two adjacent blocks.
It case the difference is too big, the how can't process a proper transition and the result looks terraced.

To avoid this you might want to extend the way between those two levels.
For example if you want to build a way up a cliff or a mountain, there's a good chance the height difference too big in order to built a nice flat way up.
In that case build serpentines, like you'd do in reality.

If a way becomes too bumby, use zhe pickaxe to remove the bumbs and the how to flatten it again afterwards