When we write metaphors, the words are usually related. Much like in music when we play chords/notes that are related (called keys). Hoewever, when reffering to words, it is called 'diatonic' relationship.
Let's use 'Tide' as an example:
These words are all related, so 'tide' becomes the fundamental tone.
Questions to ask when writing a metaphor
What characteristics does my idea have?
What else has those Characteristics?
Remember, you are looking for commanalities.
(i.e. "plenty more fish in the sea, but that girl was the best catch in 50,000 leagues." The metaphor here is a play on the well known 'plenty more fish in the sea' but the writer doesn't want other 'fish' he wants the girl he had. Hence why she was the 'best catch' THE LINK IS BETWEEN FISH, CATCH (which also means romantic partner/prospective partner) and 50,000 LEAGUES the commonality is both water and love/heartbreak)
The difference between a metaphor and a simile and
Similie = Uses 'like' or 'as'. Not quite as serious as a metaphor. Tends to regard the object as the most important thing in the comparison. Use when you have a lot of ideas.
Metaphor = Makes an object sound like it IS something else. Usually creates a dramatic confrontation or conflict. Takes it's second term (the comparison) very seriously; you cannot change this part way through the song. use when you have only one idea.
Note: The best way to create a metaphor is by combining nouns & verbs. Verbs are the amplifiers of language.