1. Look your word up in a hard copy or online dictionary.

2. Don't go solely by the definition. Pay special attention to any sample sentences that use your word.

3. If you look your word up in a thesaurus to find synonyms, make sure you also look up the synonyms and look up examples of how they are used. Synonyms often have slightly different usage rules and can't be used in the same contexts.

4. Look up sample sentences that use your word in Google Books. Scan through the list of published, edited sentences to get a sense of how your word is most often used in context.

Prepositions and Idioms

1. Look up the word the preposition goes with or one word of the idiom on Google Books. Then quickly scan the sample sentences with this word to see which prepositions or other words commonly appear with it and what the whole phrase means in context.

2. Look up the phrase you think might be correct on Google Books. For example, if you want to know whether to write "stand up for" or "stand up with," look them both up and read sample sentences with both.

3. Refer to sections 18d (standard idioms) and 31 (Prepositions and Idioms) in Rules for Writers.

4. Look up your word or idiom in the Free Dictionary online. This dictionary gives many examples and word combinations.

5. Refer to this comprehensive explanation of how prepositions are used in English.