As teachers, we obviously have to give a lot of instructions all the time and I have observed two ways of doing this. The first is that teachers start raising their voice and try to get the students attention that way. Often instructions have to be repeated, even more than once. When students don't listen, the teacher's voice gets louder and louder. Yet most often this has the opposite effect - students start speaking louder to each other and don't seem to listen at all.
The second way is much preferred. The teacher makes sure that he/she has the attention of everyone in the class first. This can be done by raising hands until everyone is quiet and focused on you. It can also be done by standing in front of the class and making some kind of noise, or ringing a bell, and wait until it is quiet. Then give instructions ONCE ONLY.
The "once only" is important! When the students get used to the fact that you will give instructions only one time, they are more likely to listen carefully. If they don't listen and ask you to repeat, you can simply refuse and tell them to ask their neighbor. After a while, everyone gets tired of having to repeat the teacher's instructions for their classmates and so the students will push each other to listen. However, if they don't understand the instructions the first time, they can ask of course.
The great advantage for you is that you can spare your voice and you don't have to exhaust yourself getting louder and louder.
Please feel free to share your experiences with giving instructions here.