As a living language, the rules of acceptable English usage constantly change.
While the only correct way to make possessive a singular noun or name ending in s was once to add an apostrophe (Francis' hat, my boss' temper, Lars' cottage), that is no longer the case. It's fully acceptable now to make those nouns and names possessive the same way you would any other, with apostrophe-s: Francis's hat, my boss's temper, Lars's cottage. The only exceptions are Jesus and the Greek and Roman gods whose names end in s. Those still get just the apostrophe.
As Jamesaritchie says, you will not be wrong if you go old-school, but you're free to embrace the newer practice if your prefer, so long as you're consistent and do not violate any style guidelines a publication makes available to its writers.
Maryn, solid on this
There's one other exception that has always been allowed. When a singular noun ends in s, and the next word starts with an s, you're allowed to drop the s after the apostrophe, new school or old.
Something such as Francis's sister brings up the mental image of a snake hissing.
Place names are also a usual exception in the United States, and in most Federal writing.
And Jesus, yes, but also Moses.
I read somewhere, Wikipedia, I think, that when you look at the writing they hand down in decisions, even the Supreme Court is divided five to four in favor of keeping the additional s, unless one of the exceptions in brought into play. One new Justice, and we might have a reversal.
What swayed me was consistency. Enough exceptions exist that I found consistency was pretty much impossible if I kept the additional s. I like my writing to be as consistent as possible, so no additional s for me.