You need to check that the students have understood because they are unlikely to tell you if they haven’t. If you say "OK?" , they will all nod and say "OK". Then they will simply bumble through the exercise, doing it wrong, probably aware that they are doing it wrong, and losing confidence.
Use words like ‘Is that clear?’ rather than ‘Do you understand?’ . Why? Because the chances of a student saying ‘No, I don’t understand’ are very slim. If someone asks you that question, it has a tendency to make you feel very stupid. Would you admit to not understanding something in front of others, especially in a classroom situation? I wouldn’t!
The student who doesn’t understand will be convinced s/he is the only one who doesn’t get it and will not want to admit that in public.
Using the words ‘Is that clear?’ shifts the blame to the quality of the instructions instead.
Here’s a simple way to ask and check.
1. Ask your students ‘Is that clear?
2. If it’s clear, fine. If anyone says "No, can you explain that?Can you explain again?"
DON’T.Ask if one of the other students can explain it.
3. If nobody understands it, go through an example step by step together. They should get it then.
4. If they still don’t get it, go through another example together.
5. If the poor things are still lost either 1) Do the whole activity together as a class, if possible or 2) Table it for now and go to the next activity. Figure out another way to explain this next time.
6. Another way to check understanding of instructions is to ask the students to imagine that you are a new student who has just come in - can they explain how to do the activity?
The most important thing is making sure they are really understanding what you are saying and not just nodding to please you.