Hi Bart,
First off,,, have you already gluded the interior piece in yet? If not,,, here's the way I do it. First off, I plough out an area about 1/2" wide, at the fold area on the inside of the outside (tooled) cover. I do this for a couple reasons, it helps the cover to fold over easier,,, & also, since that will be the last area, that will be stuck together, it will pull the inside piece slightly tighter there. Also, If you are using light leather for the outside.. 4oz-6oz. You may need a stiffener, on the inside also.
Usually the inside piece will be a little smaller than the outside cover, so that when You are gluing them together, you are also stretching the inside to fit. On a cover where the total width is about 14",,, the inside will be as much as 1" smaller. It all depends on how stretchy the inside cover material is. I always cut my inside cover about 2" bigger all the way around,, just for a "fudge" factor,,, just incase I don't get things alligned correctly during the gluing. Cover both of the mating surfaces with a good contact cement. Allow both pieces to dry completely,,, about 10 mins should be fine. On big pieces, I will put wax paper, in the areas where I want to make sure they dont stick together yet. Now, start by adhearing the center of one edge, so that when you start stretching, you will be pulling across the fold area,,, not in the same direction as the fold. Now, with that edge stuck, start stretching, & sticking, the center, until you get to the other edge. At this point, the whole cover, is wanting to curl up. Now, start streching, the unstuch areas, down, & toward the corners. Do this all the way around. At this point, the cover, is wanting, to partially fold, on it's own. Now, hold the piece as flat as possible, & press down & stick the fold area. Once you do all of this, You shold get a cover that when folded, has no wrinkles,,, or very slight wrinkles, in the fold area.
This sounds like a lot,,, but it's really pretty simple. Good luck,,, & remember, with contact cement, once it touches,,, it's pretty well stuck!!
Ed the"BearMan"