Todd Spangler NY Digital EditorThe Duffer Brothers are going to take aspiring TV creators behind the scenes in the Upside Down.Matt and Ross Duffer, who are the creators, writers and executive producers of Netflix smash hit “Stranger Things,” launched a class on celeb-focused online learning platform MasterClass about how to develop an original TV series. The debut of their class comes on the eve of the July 1 premiere of “Stranger Things 4” Volume 2 — the final two episodes of the season.The Duffer Brothers’ class comprises 18 video lessons, totaling 5 hours and 18 minutes of instruction.