Theory:

Two-word proper nouns can also be classified as compound nouns. Proper nouns are name, specific people, places, and things.
Example:
1. Costa Rica
2. Arctic Ocean
3. Pisa Tower
4. Swami Vivekananda
When a compound noun is a single word, make it plural by adding "s" to the end. If the compound noun is hyphenated or composed of two separate words, remember to add "s" only to the word that is plural.
Example:
1. One sister-in-law ⇒ two sisters-in-law
(There are two sisters, not two laws.)
 
2. One Commander-in-chief ⇒ two Commanders-in-chief
(There are two commanders, not two chiefs.)
Plural and possessive rules for compound nouns:
In hyphenated words, usually, the "s" goes at the end of the noun.
Example:
1. daughters-in-law
2. go-betweens  (Sometimes the "s" is at the end)
3. higher-ups
In the open form, the "s" is often added to the noun. 
Example:
1. assistant secretaries of state
2. full moons
To make a compound word possessive, you usually add apostrophe + s to the end of the word.
Example:
1. Mother-in-law's ring
2. Six-year-old's birthday
Compound nouns  plural:
Example:
1. handful - handfuls
2. mouthful - mouthfuls
3. spoonful - spoonfuls
4. bucketful - bucketfuls
5. cupful - cupfuls
6. truckful - truckfuls
Important!
Some hyphens are omitted over time.