I think you would pin the backing to both leaders, then lay the batting on top, then lay the quilt top on top of the whole thing and secure it with pins - basically the same as floating the quilt on a longarm, except it's a much deeper frame and you don't have rollers. I'm not sure how you are supposed to get to the middle of the quilt to hand quilt it though, and I think you would have to adjust the leaders every time you loaded another quilt. I'm not sure any of this makes sense.
Here's an older post that might be helpful.
How do you attache quilt to old fashioned quilting frames?