Question
could you please tell me how you divide a mature rose ball.

Answer
Hi
Why would you want to divide a nice mature rose ball?
You won't get two if you do, you know that right?
If it is too big for that spot then it is best to move it. Simply pulling it out, making the root ball smaller and then planting it back will only give you a little time before it is too big again.
So just tell me whats the problem and I will see if I can help?