Spent about 10 hours on the car today. Perfect day for installing a headliner - about 85 and humid.
I bought the new headliner about a year ago and of course they come in a box all folded up. After pinning it to the wall and letting it hang for about a month so the folds would come out, I then used the wife's steam iron and got the rest of the major fold marks out of it. After it was nice and flat, I rolled it around a 5" diameter, five foot long piece of PVC pipe and wrapped the whole thing in a couple blankets and stored it inside. It worked great - a year later and no wrinkles when I unrolled it.
Here's the finished product in the car:
