Theory:

In the study of idioms, it is important to know the usage of verbs and nouns as idioms, when followed by suitable prepositions.
What are prepositions?
Prepositions are words that are used to link nouns, pronouns, phrases. It generally shows location, direction or time. Examples - in, of, down, from, over, above, under, etc.
Sometimes when we combine noun/verb with prepositions we acquire idioms or phrases with idiomatic sense.
Noun + Preposition = Idiom
Example:
"Cave" is a noun. When a preposition "in" is suffixed, "cave in" is the new idiom formed.
The "cave" is a chamber in a mountain or a cliff. "Cave in" means to collapse under pressure.  
Important!
Check if a different and newer meaning is derived, when idioms are formed!
Verb + Preposition = Idiom
Example:
"Break" is a verb. When the preposition "in" is suffixed, "break in" is formed, with a new meaning.
The "break" means to make something into halves/pieces. "Break in" means to forcibly enter a building.
Important!
Check if a different and newer meaning is derived, when idioms are formed!
Some verb-preposition combinations that can be used as idioms, with meanings:
  
Verb + Preposition
Meaning
Fall behindfail to keep up pace
Blow upexplode
Cry outprotest
Cut downreduce
Back upsupport
Thrown out
rejected
 
Some noun-preposition combinations that can be used as idioms, with meanings:
 
Noun + Preposition
Meaning
Back offwithdraw
Rock bottomlowest possible range
Round offto bring to completion/symmetry
Set outBegin a journey
Nail itGet it perfect
Break downSuddenly cease to function
 
Thus, many combinations can be made to acquire idioms.
 
Important!
Remember, idioms should have a different meaning than the literal meaning.