I was asked this question recently. So, I sat back and tried to think how I, myself, have done so in the past, and how I have responded to those who have asked me.
The answer I came up with is that I have never actually asked anybody to be my mentor, and almost all people who I have tried to help have never asked me to be their mentor.
What I have done is to ask a lawyer with an expertise that I did not possess if I can ask him or her a question. Then a follow up with other questions. I get use to asking and following up, and they get use to answering. I have respected the advice and they have appreciated me asking. Then a business friendship develops, and a sort of mentorship is born.
Instead of asking such a doltish question of will you be my mentor, it is better to think of penetrating and informative questions that you need answered.
Besides, asking someone to be your mentor is kind of like asking them to be your parent. It is a big thing to ask and a difficult thing to accept. You might as well ask them to take and raise you. But, essentially asking them if you can call with questions or difficulties when the need arises, then why not. There is nothing difficult in that.