Letters of Recommendation for Law School

I requested two of my letters of recommendation for law school last night. One is my old department chair and one is a professor. They both know me really well from my work with our professional society so they have seen both my in class performance and my leadership/volunteer work. I was thrilled that they were both so enthusiastic to write letters for me. I'll be sending them my resume and personal statement from last year soon to give them some material to work off of. I really want to get good letters though and these being engineering professors I'm afraid they'll write a MS program reference. "This is a good student, I heartily recommend them to your program and you should believe me become I'm a really impressive academic that you should respect as your peer." This isn't what I think law schools are looking for. An official letter on official stationary from an academic is probably less meaningful that thoughtful commentary on my strengths. I'm trying to figure out how to phrase my letter request to solicit the "right" kind of letter.

The other one that really freaks me out is my third letter. This professor/friend said, "Just write the letter and give it to me to sign." I don't really feel comfortable doing that, but it really seems like an opportunity. Any advice?