Your child is angry because his sister came into his room. He throws a toy at her and you respond with, "Use your words!" What was the result? Did he shift gears and go into a verbal explanation of what he needed or why he was upset? Probably not and it's probably because he didn't know what words you wanted him to use.
As parents we're quick to tell our children, "Use your words" but we've probably not taken the time to teach them exactly what words we want them to use. Sometimes it's a matter of modeling those words for your child. Think about what your child sees when you're upset. Does she see you throwing something or yelling or does she hear you using your words? Put your own emotions into words and your child will pick up on what words you want him to use.
Find opportunities to work with your child to find the right words to describe emotions and how to communicate those emotions. Trace around a gingerbread cookie cutter. Have your child color in parts of the person that represent where we feel emotions. For example, when we're sad our heart may feel blue. When feeling angry our face might look and feel red hot. Point out how other people are feeling. Use characters in books or TV shows.
If your child has siblings use their interactions to teach what words to use. For example, when your son hits his little sister because she knocked down his block tower, tell him what you want to hear. Say, "It really made you mad when she knocked that tower down but you can't hit your sister. You need to tell her that it makes you angry." Have your child repeat the words so he will grow used to what it is he needs to say.
In order to teach your children how to use their words you need to be specific about what you want your children to say. You need to model the exact words repeatedly and find ways for your child to practice using them.