The only way I know to make slopes like this from within the game is:
1) Place terrain blocks
2) Beat on the terrain blocks with a combination of low-level stone shovel, stone pick, hammer, and/or fists to pound it approximately into the shape you want
Making something as smooth as your picture would take a long time. I've only used the technique to repair holes I've created accidentally, where there was a bump after placing a soil block to fill the hole. I just beat the bump flat (so it ends up as a partially-damaged soil block, but even with the surrounding natural terrain).