Adding screen flash (Hitfilm 3 Express) I want to use some kind of effect, to make the video flash for a short time, when something happens. (Example for a gaming video, when you get a kill the screen should flash for like a half a second)

I am using Hitfilm 3 Express. So far i found an effect, which looks pretty good, but i have no idea how to make it look a little more smooth. Is there a way to use effects over time in Hitfilm 3 Express?
You need to switch to the compositing timeline in HitFilm so that you can animate your effect (using keyframes). There are 2 timelines in HitFilm - one for editing your footage and another for layered animation work.

It's worth watching the tutorials on the HitFilm YouTube channel here > https://www.youtube.com/user/FXhomeHitFilm

This specific tutorial explains the difference between the two timelines > https://youtu.be/bawBXIhMxsM

This video explains animating with keyframes > https://youtu.be/jEPm2FPMZI8

I hope that helps! :D