A thesaurus—a collection of synonyms and antonyms—can be an excellent way of improving word choice in your document.

The thesaurus provides a variety of word choices as suggested replacements that may help you better describe a feeling, a landscape or an event. This can be particularly useful if you have a tendency to overuse the same adjectives, verbs and nouns or are simply stuck for a word.
Here are some examples:
Sad – morose, gloomy, miserable
Small – miniscule, petite, diminutive
Thin – emaciated, skeletal, slender
Pretty – gorgeous, attractive, striking
Baby – infant, child, toddler
Walk – stroll, saunter, meander
Abandoned – alone, forgotten, forsaken, vacant

Hence the thesaurus will enable you to use more descriptive words and improve word choice while avoiding repetition in your writing. You can also use the options offered to make further searches. The thesaurus can also help improve your vocabulary and comprehension of words.
While the use of a thesaurus can be a lifesaver don’t make the mistake of overusing it. An incorrect word choice could have the opposite effect on your work and instead of improving it make it sound amateurish.

You will soon realize that not all words are interchangeable in a thesaurus so tread carefully. While the variations in English make it the rich language that it is, it can trip you up as well as words have nuances and specific contexts in which they are used.

If you are unsure about the meaning of a word, or it is not familiar to you, it may be wise to consult a dictionary while using the thesaurus.

You should also consider the synonym in the context that you are using it. For example you may wish to avoid repeat use of the word “walk” but “saunter” might not be the right word choice for really young children whereas “amble” or “stroll” would be appropriate.

Similarly if you are talking about an “abandoned” item of clothing it would be okay to say it was “cast aside”, “forgotten”, “dropped” or “dumped” but you would not use words like “vacant” or “unoccupied” which are also synonyms for the same word and may better refer to a sentence that talked about an abandoned house, park, lot etc.

Using large or complicated-sounding words from the thesaurus that aren’t used in conversational language can also make your text rather awkward. If you are not a native English speaker you are better to use more simple words that you understand and may even have heard people use.

The thesaurus will also list the antonym; words that are the opposite in meaning to the entry word. As with the synonyms these are not always the exact opposite and may simply be “contrasted words” so consult a dictionary when using the antonyms also.

There are a number of ways to utilise a thesaurus. In Microsoft Word there is a thesaurus built into the program. You may like to keep this open while writing your document and using it as a word prompter from time to time.
Alternatively you may wish to use a web resource such as http://thesaurus.com. On this website you simply enter your word and the site provides a definition, the part of speech it refers to (noun, verb, adjective etc) and a variety of synonyms and even phrases you could use instead.

This list of synonyms is sorted with the more commonly used words appearing at the top and the lesser used ones at the bottom. There are also examples beneath the word choices showing how the original word is used in a sentence.
The definition of the word is particularly helpful as it provides a reference point and allows you to evaluate the synonyms based on it.