1. Normally the last word is made plural; e.g.

BREAK-IN = an illegal forced entry of a building or vehicle, typically to steal something. FOOT-MAN = is a male domestic worker

NOTE-I: But where MAN and WOMAN is prefixed, plurals are formed differently; e.g.

a) When both the nouns are main nouns, both nouns are made plural; e.g.

i) man driver — men drivers
ii) woman doctor — women doctors

b) When only the second noun is main noun, the second noun is made plural; e.g.

i) man eater — man eaters
ii) woman lover — woman lovers
iii) man hater — man haters

NOTE-II: When the first word is made plural; e.g.

i) court-martial – courts-martial (COURT-MARTIAL = a judicial court for trying members of the armed services accused of offences against military law)

ii) Knight-Errant — Knights-Errant (KNIGHT-ERRANT =  a medieval knight who travelled around doing brave things and helping people who were in trouble

2. With compounds forms of ‘Verb + er, Nouns + adverbs’ and ‘Noun + preposition + noun’ the first word is made plural w; e.g. 

a) VERB+ER, NOUNS + ADVERBS

i) hanger-on — hangers-on (HANGER-ON = a person who tries to be friendly and spend time with rich and important people, especially to get an advantage; e.g.

Wherever there is royalty, there are always hangers-on)

ii) looker-on  — lookers-on (LOOKER-ON = someone who watches something that is happening in a public place but is not involved in it; e.g.

At every gathering for dances, sports, or games of any kind there are more lookers-on than participants.

iii) runner-up — runners-up
iv) passer-by — passers-by

b) NOUN + PREPOSITION + NOUN

i) coat-of-mail — coats-of-mail (COAT-OF-MAIL = a jacket covered with or composed of metal rings or plates, serving as armour)
ii) Commander-in-Chief — Commanders-in-Chief
iii) lady-in-waiting –ladies-in-waiting (LADY-IN-WAITING = a woman whose job is to help a queen or other woman of high social position)

iv) man-of-war — men-of-war (MAN-OF-WAR = an armed sailing ship)
v) sister-in-law — sisters-in-law
vi) ward-of-court — wards of court (WARD-OF-COURT =a minor or one legally incapable of managing his own affairs, placed under the control or protection of a guardian or of a court; e.g. Rohit was made a ward of court.)

NOTE: We say spoonfuls and handfuls, because spoonful and handful are regarded as one word.

3. Initials can also be made plural; e.g.

i) MP – MPs (Members of Parliament)
ii) VIP – VIPs (very important persons)
iii) OAP – OAPs (old age pensioners)
iv) BA – BAs
v) ATM – ATMs