With Projects, the best thing to do is to find the Project Page or Badge List and check the instructions on who to contact to join.
I will admit, I've not been a part of this before, but here's the instructions:
https://www.wikitree.com/index.php?title=Special:Badges&b=sourcerer
To earn the badge, you have to participate (according to the information here) in the Sourceror's Challenge. It looks like they just held one for September, so you will have to wait for the October challenge if I'm reading this correctly.
Perhaps a Sourceror will come out and correct me if I'm wrong.