Maybe I'm reading the post wrong but it seems to me that you're asking how to make a quilt "sandwich". If I'm wrong, just ignore me. I tend to blather.
First thing is to IRON. Iron your backing and top. Put your back down wrong side up (use a table, floor, whatever). Make sure it has all wrinkles out. Clamp it down, tape it down, tack it down, any method that will keep the backing from shifting on you.
Put your batting down next. A lot of people put their batting in the dryer for a few minutes to get the wrinkles out, your choice. But make sure when you lay it out that you do not have any wrinkles (wrinkles are like a deadly sin and will fowl up your world). You can adjust this over your backing so that you get the "right" fit (which is why you tacked down the backing - you don't have to keep adjusting the backing then) Once I get the backing down where I want it, I pin the corners in place, but maybe that's just me.
The next step is to lay out your quilt top. Figure out the middle and put it on top of the batting right side up on the middle of the batting. Again, get your deadly wrinkles out. Then just pin your quilt to stabilize your sandwich. I usually pin about 2" apart.
Some people use a spray when they put their layers together. Your choice.
Yes, at this stage you will have raw edges. After you finish quilting, the you put on a binding and voila, no more raw edges!
As for "stitch in a ditch" goes, I never could get the hang of that. I ended up getting a foot for my machine that does the stitch in a ditch. That and my 1/4" foot were my best investments!
Hope this answers your questions.