The headliner is stapled to the sides on the right and left, and there are triangular cut outs on a piece of metal under the rear cargo area panel which you stretch the headliner and the hooks grab and cut through to hold it in place. At the windshield area, the windshield needs to be pulled and it wraps around the upper lip of the windshield frame. Be sure to seal the corners of the windshield really well upon re-install, those are the most common areas to have pooling and leaks.
You will need to get a new windshield gasket, steel rubber carries it, about $96 shipped with a charter member 10% discount, or around $110 shipped without.
I just did this in my '73 wagon I have some pictures if you need some visual.