As stated by @Relyss, the DevTools (or debugtools, as they seem to call it - my bad) are already available in the Mods-tab within your Stonehearth settings. All you have to do is to activate them.
I have found these tools handy when it came to “correcting” the last details of my added models.
When it comes to the act of modding itself, I would suggest you to download the official example mod and go from there:
By studying these files you will get a basic graps of how to add new entities and recipes. To do more advance stuff, you could learn from what already exists within your copy of Stonehearth.
Within _…\steamapps\common\Stonehearth\mods_, create a copy of the original stonehearth.smod.
Rename it to stonehearth.zip and unzip it - this will now yeild a folder namned stonehearth.
Now, you can freely browse this folder and everything found within regular Stonehearth - it makes an excellent way of learning in my opinion
And, if you ever get stuck, it seems to me there are a lot of talented people on the forums - so ask away and the help shall arive very soon I believe
I, too, will to what I can to help if I see any questions from you.
Best of luck now - can’t wait to see what you’ll produce!
Edit: Oh! When working with JSON-files it can be tricky to see if everything is correct. At least for me; I do all my work in Notepad++ xD
So, I will also provide you a link to an online JSON verifier.