On a level/ at a level

[Ahmedabbas]

How to use them?

 

[Tarheel]

How should I use them?

I don't know. What do you want to say?

 

[emsr2d2]

How to do we use them "on a level" and "at a level"?

Note my corrections above. We don't start questions with "How to". Please put the words you're asking us about in the main body of your post, not just in your title.
Why are you asking this question? What are you trying to say? Please give us at least one full sentence in which you don't know which one to choose.