During therapy with young children with delays as well as with my interactions with my own son, I purposefully keep my sentences simple and repetitious. When rolling the ball back and forth to a child, I may say the word "ball" 20 times or more. I also pause to let the child have a chance to say it or go after it if it rolled away. This is important because the brain may take multiple times to make that connection that the sound of "ball" matches the actual object. So, then next time the child wants to play ball, they just have to say it. Some kids with disabilities may use sign language or pictures instead of words, but even then, they need repetition of those approaches too. Also, if I start the activity out the next time by saying "Get the ball",...pause..."ball", I hope that the child goes to retrieve it. If not, then I help them go get it, and hope that the next time the child will remember what a ball is.
In addition to repeating the words multiple times, the other big thing to remember is keep the sentences simple. I don't have to describe every detail of an activity especially since the child doesn't understand all of those words. That also increases the chance that the child will tune me out. But if I keep is short, I am more likely to gain the child's attention.