I used to help my grandfather do it. It was many years ago. We took a big galvanized funnel and stuffed the bottom with course steel wool. Filled the funnel with a mixture of oak and maple ashes. My grandfather said that maple had a lot of lye but we burned more oak. We had the funnel mounted with a glass bowl under it and had a hose mounted above the funnel. The hose was left dripping, steady drip, but not running. We would adjust the water flow so that it stayed very wet, but never overflowed. The liquid that came out of the bottom was strained through old burlap sacks and then used in the soap.
I have no idea the lye content or anything like that. I do know the liquid would burn my hands if I got any on them and didn't wash it off quickly, but that was 40 years ago. I hope that helps at least some.