This is a very simple idea but we find that our students have a lot of problems doing it.
So the explanation is very short but we are giving a lot of practice activities.
If you use a negative adjective, it can sound aggressive and insulting.
You are small.
He is stupid.
It is often much better to use not very plus a positive adjective.
You are not very big.
It is not very clean.
He is not very intelligent.
As I said, a simple idea that can make you sound much more professional.