Many people struggle with this particular aspect of punctuation. There is no need to panic. There is a simple, straightforward rule for deciding where to put the apostrophe.
Possessive apostrophes are used to indicate that something (an item or a quality) is possessed by somebody (or something).
Sometimes, the possession is not as clear as it might be. If there is any doubt about who or what possesses what; do not write anything, say it ... and listen for the "s".
Today's date: today possesses the date. (The date belongs to today.)
The dog's owner: the dog possesses an owner (even though the owner bought the dog!)
The burglar's swag: the burglar possesses the swag (even though he stole it!)
The dogs owners? (apostrophe omitted)
Both dogs and owners end with "s"! No need to worry. In the English language, the possessor/owner comes first in the word order, so it is easy to work out what belongs to whom.
The dog's owners. (Because the dog, coming first in the word order, is the possessor of several owners.)
Once you have identified the possessor, place the apostrophe (and the "s") immediately after the possessor.
IF: One girl has one dress.
THEN: The girl is the possessor
SO: The girl's dress
IF: One girl has lots of dresses.
THEN: The girl is the possessor.
SO: The girl's dresses.
IF: A number of girls have a number of dresses.
THEN: The girls are the possessors.
SO: The girls's dresses. (However, when confronted with "s's" we drop the final "s"
SO: The girls' dresses.
IF: One woman has one dress.
THEN: The woman is the possessor.
SO: The woman's dress.
(Obviously, we would not expect to encounter womans without an apostrophe.)
IF: A number of women own a number of dresses.
THEN: The women are the possessors.
SO: The women's dresses.
Women is already plural - but still needs the possessive apostrophe and the final "s". (Obviously, we would not expect to encounter womens without an apostrophe.)
You should note an important EXCEPTION to this rule. When the possessor is merely identified as it, no apostrophe is used. (It's always means it is.)
I saw the car but I am not sure of its colour.
Sue adored the house and its lovely garden.
Introduction - Capital letters - Full stops - Question Marks - Exclamation marks - Abbreviations - Contractions - Commas - Speech marks (Quotation marks - Inverted comas) - Possessive apostrophes - Colons - Semi-colons - Brackets - Hyphens - Dashes - Obliques (slashes)