Compound nouns can be formed with:
Noun + noun
Package holiday, computer game, TV set, website
-ing + noun
dining room
adjective + noun
greenhouse
to form the plural of these compound nouns, we use the plural form of the (second) noun.
Package holidays, computer games, TV sets, website, dining rooms, greenhouses
noun + preposition
passer-by
noun + preposition + noun
sister-in-law
to form the plural of these compound nouns, we use the plural of the (first) noun.
passers-by, sister-in-law
other combinations
take-off
to form the plural of these compound nouns, we add –s to the last word.
take-offs
lucky0101