First mark-cut it and then place it because afterwards, like kneeling, it cannot be cut at all and you will also cause damage.
The cut is better done with a saw, straight it does not cut it down at once.
Because the distance tile-bottom of the door, that is the = cannot be 100% the same from one end to the other, expect at some point the silicone may possibly find, so in the open-close of the door it may make a subtle sound.