As you finished the last mission, you are now back at the surface of the blue central area.
Even if you know another voxel game, you'll have to learn the very particular logic of Blackvoxel. To favor automation logic, most machines are working with direct world interactions rather than "Windows where human can click" logic.
In this mission, you'll use your "base machine" for building a melting furnace. The base machine is one of the only stuff saved from the crash. You'll find it in your inventory.
When you want to build something in blackvoxel, the game manual will give you some manufacturing instructions.
Here are the manufacturing instructions of the Melting Furnace as described in the manual.
Then, place EXACTLY 8 Blackrock green next to any side of the base machine, one by one(See the image bellow). The machine will swallow them each time you place a block (of course, you can't see it in the image) :
Finally place a blackrock blue next to one side of the machine to terminate the procedure. The Blackrock blue act as a validator and must always be put in last.
At this point, congratulations a melting furnace will get out of the base machine.
In case where something goes wrong, the most probable cause is that you
made a mistake on the kind of block or quantities you put in the
machine. In case you want the machine to return blocks you put in it,
put blackrock blue next to it until the machine get in output mode.
Once you got the melting furnace, this mission is over. See you to the next mission...