Take the Skeleton Key Mold and 2 Thorium Bars to the top of Fire Plume Ridge in Un'Goro Crater. Use the Skeleton Key Mold by the lava lake to forge the Unfinished Skeleton Key. Bring the Unfinished Skeleton Key to Apothecary Dithers at the Bulwark, Western Plaguelands.
You need two bars of thorium and a powerfully hot heat source to forge the key stem. You're on your own for the thorium, but I can help with the heat.
To our west in Un'Goro Crater is Fire Plume Ridge - a simmering volcano. The lava lake up top is hot enough to fuse thorium and bone together. Your gold has bought you a mold that simply requires you to dip it in the lake up top; the lava will take care of the rest.
That's it on my end. To finish your key, you'll need to talk to Dithers again.