Well, the last time I got them replaced, I watched the technician do it and he had to use this long ass screwdriver to guide the belt to where it had to go, and he did it from underneath the car. Hope that helps some.
I remember him loosening some kind of tensioner or something, then he had to struggle with the darn belt for like over 10 minutes before he could get it back on. It wasn't simple but he did manage to do it. I think all in all, I got both belts replaced with labor was about $50 dollars or so total.