It's not a difficult job to do, possible on your own but easier with an assistant.
The seal has 3 slots, as stated before make sure you get it the right way round.
I did crack mine refitting it, but I was rushing and it was a cold day (and starting to get dark) I hit the seal at top of screen near mirror with the heel of my hand to seat the seal and it cracked, but on later inspection there was a tiny bullseye just where I had hit it.
I was changing a perished seal, the screen wasn't great anyway, it had signs of water ingress in the lower corners and would have needed replacing anyway, but I was hoping to get it back in and upgrade to a heated one at a later date.
If you do go ahead leave yourself plenty of time.
