With the mastic a bit tacky, carefully put the carpet in, slipping it neatly behind the headliner rails so you can't see the edges of the carpet. Smooth it down starting in the middle. At this point hopefully it will stick in place. If you want to be sure, put in some of the hoops from the old head liner (you saved those, right?). Stuff some shims between the hoops and the carpet to hold it up. The photo below is of the carpet in the rear of the roof being helped by the hoops and some shims of folded foam-core. This was a pretty easy job.