After the sides are installed a hair dryer will be your best friend to remove any wrinkles. Did you buy your headliner locally or was it one of those pre-made ones I see advertised on eBay?
I bought it from Ebay. They were good to deal with and had fast service.
Since my skills in mounting the headliner is poor my friend Mattias who did my DartCharger will do this one too.
I will get some info and pictures for you. Have you bought one for your wagon?