How many sites have told you to close apps?
Those sites are wrong. Plain and simple.
The iPhone does a great job of managing apps that are sleeping and each time you begin closing items you are doing more damage to your battery life. Don’t do it.
iOS handles opening, closing and sleeping apps very well but each time you close an app and then later fire it up you are taking more battery life. Just leave them open and the iPhone will do a better job of managing it for you.