For most words, if they are singular (they refer to one thing only), the rule is easy: add ’s. This applies whatever the final letter of the word is: Susan’s advice, the box’s lid, jazz’s beat, the bus’s driver.
Remember: the apostrophe is nothing to do with making something plural (more than one) or singular; it is establishing a relationship (of possession, or some other close association) between two things. (The box doesn’t ‘own’ a lid, for example, but we are saying that the lid is associated with the particular box).
The one exception to this is some individual names (of people, places, organisations and similar things), and I’ll deal with those in my post next week.
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