How do i get my chain tensonier to stay in the exact position so that my chain wont fall off?

hey guys, im a really beginner when it comes to this, but i have gotten down to one of my last problems. basically, my chain tensioner moves out of place whenever i finally get it lined up just perfect and attempt to tighten it down. this seems like the only way to do it so that the chain wont fall off. also, would just shortening the chain work better? Thanks!
Tighten the bolts
 
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