If you have Stonehearth running, start by terminating Stonehearth ^^
Within you Stonehearth-folder (…/Steam/steamapps/common/Stonehearth), there is a mods-folder.
Place the .smod-file (from this mod or any other mod you want) within your mods-folder and now start Stonehearth.
When Stonehearth is started and you are greeted by the start screen, in your top-right there will be a settings-button.
Click it, and head over to the Mods-tab; this is where you select which mods are active and which are not