The reason it's tricky to tell you how to thread a Singer 6212c sewing machine is that it really requires a diagram to explain the process... however I think I've found something even more helpful!
By simply searching for "Singer 6212" (omitting the "c") I was able to find a detailed manual for the sewing machine.
To view it, just click here.
Some versions online are paid for downloads, but trust me, the English version of this one is free.
I downloaded it to test and it worked absolutely fine.
I don't know if this will help but I have also included a video of a machine being threaded. I doubt they are all the same but hopefully it will give you more idea of what the manual is telling you to do.