Write the sentence in your own words. Do not open a thesaurus before you write the sentence or when you are in the middle of the sentence, even if there's a word on the tip of your tongue and you can't think of it. Start out with your own words to come closest to the meaning you are trying to convey. For example "Craig runs up the hill."
Underline any word or words in your sentence that sound weak or not quite right. Look those words up in the thesaurus. Some books are organized by topic, in which case use the alphabetical index in the back to find your word. An online thesaurus will allow you to search for specific words. If you want to convey the urgency of Craig's running and the height of the hill, for example, look up "run" and "hill."
Choose words you know from the thesaurus offerings that match your meaning better than the word you wrote. Choose a word you recognize. If you recognize what seems like a good word but only know it vaguely, look it up in the dictionary before putting it in your sentence to be sure your intended meaning is correct. Use replacement words, for example, "Craig races to the top of the bluff."