Not only do you want to hem your panels and lining before attaching, you want your lining to be about 3" (unfinished) narrower than your dec fabric. It really is as easy as you think. If you/requester are planning for rod pockets, those are easy as well...just leaving an opening about double the width of the rod. General rule of thumb is double the window measurement for 'normal' fullness or triple window width for lots of fullness. I find double is more than sufficient.