An apostrophe is used for two purposes:
To show one or more missing letters:
Don’t (do not) They’re (they are) Claire’s (Claire is)
To show ownership:
Claire’s bucket, the children’s toys, Jesus’s disciples.
Ok, so this one depends on your style guide again. Some people prefer to leave out the second s and simply write : Jesus’ disciples.
A good rule of thumb is to examine what you would say out loud. You might say: Jesuses disciples, or Chrises pen.
However, you would never say: Studenteses Union
So when something ends in ‘s’ because it is plural, and you also want to show that something belongs to these, the apostrophe goes after the s.
The Students’ Union
The Union belonging to more than one students.
The Student’s Union
The Union belonging to one student.
This is the one exception to the above rule. When something belongs to ‘it’, there is no apostrophe: The animal ate its young.