You need to mount the trim to the car first.
I use rubber lubricant from here:
P-80 Temporary Rubber LubricantThis will not harm the rubber, and will make it slick enough to easily work into the channels.
I start on one side and have a partner support the remainder of the rubber to eliminate stress at the two corner joints (which can break easily).
I work one side pushing/pulling/stretching the rubber down the channel. Once it is nearly down, you must carefully work the corner onto the top of the channel. Once it is started, you can finish pushing/pulling/stretching the seal down the channel.
Next work the seal part way across the top (about 1/3 of the way across) This just helps support the rubber seal.
Work the second side down the channel on the opposite side the same as you did the first, being extra careful when it gets close to the corner engaging the top of the channel.
When both sides are fed down the channels, work the remainder of the seal across the top of the windshield.
Lastly, when the seal is in place, you may need to slightly crimp the channels (just as the factory did) to hold the seal in place. The last one I did, I didn't have to do this, but it was an old seal.
I am about ready to replace an old cracked seal as well, but I have not yet had the time. Maybe this weekend.......
Good luck.