The locking part of the gasket goes toward the outside of the truck. The window then goes in from the outside. Last, you have to start getting the locking strip tucked into it's proper position.
I do not know how easy or hard that is to do. However, I have watched them being installed, and one trick is to apply plenty of liquid soap to the locking area. Last one I saw done, the mechanic told me he liked to use a liquid hand cleaner like GoJo, as it is a little thicker and stays in place. Use the type without the grit in it.
1981 J10 Honcho, 360/4 spd.