I just slide the bottom in over the edge of the small lip first, then I push the top (with the gripping ridges) in next. Usually it makes a very slght "popping" sound, when it snaps into place, but not always. I just push it in with my fingers. Sometimes it's easy, sometimes it takes a little extra persuasion. It also helps to push it either left or right towards the front or back of the door to take up the "slack" as you push it back into the trough an inch or two at a time.
I was a little apprehensive about pulling it out the first time I did it, thinking it won't go back in place, but it does. It's rubber and very pliable. Just work it in, and don't be in a hurry and you shouldn't have any problems.
You know that it's back in place when you have the "griping ridges" that run along the top of the gasket tucked back in out of sight.