I wish I could tell you that if you implement everything I say in the post, you will never face a situation like this again. Since, however, one must not tell lies, the only promise I will give you is that it will decrease the frequency of such occurrences.
1. What to Do
You might want to read out the instruction from the textbook. I have been trying to cut down on this habit. My rationale is that once your students learn to be read instructions to, they stop doing it on their own. As a result, when it comes to tests, they skip the instruction and end up doing something completely different from what they were supposed to. For this reason, all I say is:
Book/page/Activity, e.g. Student's Book, page 15, activity 3a
If, however, you want them to do something that does not have a given instruction or differs from what the coursebook requires them to do, you should state it as clearly as possible.
2. Alone, Pairs or Groups
We have discussed the benefits of pairwork and the stages at which it is used. You should indicate whether your students should do the task alone, in pairs or in bigger groups. If you have an odd number of students and it is unclear how the groups should be defined, use hand gestures or their names to indicate how exactly they are going to work. This could be: "In groups of two (show the pair), two (show the pair) and three (show the trio)". You might also add: "Samvel is with Anaida; Gagik is with Hayk; Anna, Iren and Albert are together".
3. How Much Time They Have
Timing is important for several reasons:
If we want our students to develop certain subskills, such as skimming and scanning, we should limit their time, so they learn to do this quickly rather than read the entire text in detail.
It helps with the classroom management since it signals your students that they do not have all the time in the world and they had better focus on the task at hand.
How much time should you give them?
The typical timing for tasks is between two to seven minutes. It is generally recommended to use common sense to decide how much time the task will take. If you are new to this, do the task yourself and time it. Since you are, assumedly, smarter than your students, it will take you less time to do it. So add a couple of minutes. You can also ask your students how long they think it will take them to do a task and choose the average of their answers.
Bear in mind that the time is not set in stone. If you see that everyone has finished the task you might ask them: "Ready to check?" and proceed with the lesson. Likewise, if the time is up but the students are not ready you may ask: "Do you need one more minute?" and add the time.
In terms of classroom management, try to occupy the students that have finished the task before others. It is not recommended to let them move on with the other tasks. Instead, you might ask them to start doing their homework, send them a Quizlet set to revise, talk to them about their previous day and plans for the next one or check the mistakes they have made during the lesson.
The entire instruction would be something like:
Student's Book, page 10, activity 10b. Work alone. You have three minutes.
You may change the order or skip the page number if they are already working on that page.
NB It is important to use your body language and voice to indicate that you are giving an instruction and that they should pay attention: sit straight, show the task on the book, slightly raise your voice (or, if they are too noisy, and you want them to calm down, lower it), stress the numbers (page, activity, time). I also use the timer on my mobile to track the time. Since I constantly multitask, this gives me a breather and takes my mind off tracking the minutes.
Instruction Checking Questions (ICQs)
If you have already studied the structure of a language-based lesson, you should be familiar with the concept checking questions. ICQs are similar in a way that they check your students' understanding of the given instruction. Frustrating as it might sound, even if you have followed all the aforementioned steps, there will still be students asking "Which exercise?" or "Are we doing it alone?". As time goes by, you will begin to identify such students. You can address your ICQs to them directly or to the group as a whole: "Which exercise are we doing?", "Should you do it alone?" and "How much time do you have?".