When I did the wagon we discovered the hard way that you need to measure the front and back before you start attaching it...otherwise when you come up short at the end you start over.
Put in all the support hoops and attach them to the roof, then measure carefully to make sure it is centered and start at one end and work your way around. We used some thin wide spackle spatulas with a little masking tape over the sharp edge to stuff the material into the retaining teeth. You may also want to ensure that none of the teeth (not sure the technical name) are not bent out from removing the old headliner or the new one will not stay in it.
Take your time and plan on having some beer/aspirin at the end for your sore back from being in unnatural positions.
hope this helps and good luck.
-Erik