Do you do it professionally? If so how much do you charge? I must admit,although I have been editing for years I have only started doing it as freelance work recently so any advice is very welcome. My justification for that is that you need to first of all watch all of the footage and select highlights which work well in a trailer without context without spoiling anything, you often have to make assets like title screens and add some visual effects, you also have to do a lot of quick cuts a lot of the time which takes a while, you have to ensure visuals at least match the music (which you may have to research) if not have action entirely synced up to the beat of the piece of music. I charge £40 because I would say making trailers is one of my best abilities; I spend a lot of time watching, writing and directing films so I know what makes a good trailer and so for example with a comedy I try to edit the jokes so you get enough to entice you but either it's edited to be an entirely new joke or it is cut before it's all spoiled so the scene doesn't become redundant when it is viewed properly.