The plural of standard -s plural words never has an s sound added to indicate possession, so simply write an apostrophe at the end to show possession.
“The boys’ bikes all broke that day.”
The possessive of singular nouns ending in an s sound in contrast does add another s sound, so add an apostrophe and then an s in writing.
“Jesus’s words were often polemical.”
In the end, it’s a simple test of pronunciation: add an ‘s where you make an extra s sound and add only an apostrophe where you do not.