For creating the voxel models themselves, you have a few choices. Check out this topic from a few days ago:
I would not recommend starting with a new class. A craftable item is a very good place to start, at least to get your feet wet. I’d recommend looking at the Startermod on the dev teams Github. You can see how an item is added to the game, and model your mod after it. I’d also recommend opening up the game itself, by renaming stonehearth.smod to stonehearth.zip and extracting. (Make sure to copy this somewhere else, as having both stonehearth.smod and a stonehearth folder will cause issues during updates). You can then dig through all of the games files!