How do you show possesion with a last name that ends in an "s"?

The correct answer is Jones'. You put the apostrophe after the s.

msgator is correct

Putting the apostrophe after the "s" is the convention dictated by The Associated Press Stylebook, so it is commonly followed in journalism. However, The Chicago Manual of Style — widely used by book editors — recommends that most possessives of proper names include an extra "s" (e.g., "Jones's bike"). It makes explicit exceptions for Jesus, Moses, and names with more than one syllable and an unaccented ending pronounced "eez" (e.g., "Euripides' bike", not "Euripides's bike").

If your work or assignment requires you to adhere to one convention or another, then do so. Otherwise, either form is acceptable as long as it is used consistent throughout.

The older I get, the more I realize English us the most messed up language ever.

I thought I remembered from my grammar classes that the extra "s" at the end depended on whether the name/word was plural or singular. For instance, "Nah, that dog aint mine, it's Jones's," is correct because you're talking about one Jones. While "I'm going to the Jones' house for a good ol' fashioned luau," would be correct also because you are referring to the house that belongs to all of the members of the Jones family.

When words that normally don't end in "s," such as steelworker, are pluralized, they only receive one "s" when becoming possessive. Thus "steelworker's toolbox" refers to the toolbox belonging to one steelworker, while "steelworkers' pension plan" refers to the pension plan belonging to a group of steelworkers.