How do I calculate this forced mate in 8 moves I'm rated 1200?

Yavan

You don't. LOL. But seriously… the answer is slowly and with patience. The best way to learn calculation and tactics is with problems that are just a little bit above your head. Chess rewards the patient studier. What I would suggest is you get a book, or other collection of problems and concentrate on the problems you have difficulty with, and work them out. Chess databases provide a great way to do this today. (You can download SCID for free on the Internet - it's an excellent database program and it's open source https://sourceforge.net/projects/scid/). Take your time. Move back and forth in the position. Don't take the book's word as gospel. Try to see if there are other moves you could play, than those suggested by the book. (Turning on the analysis engine helps figuring out why those moves weren't considered).