A lot of that comes with experience editing and have the proper software. For example if you are a Mac person, you probably would edit in iMovie or Final Cut Pro and if you use a PC you are probably going to use Pinnacle, Sony Vegas, or other PC type software.
Through use of images, transitions, music, and such you'll get a creative feel for what looks appealing. It is a fun learning curve. You can make very easy videos with great content which is just as valuable to the purchaser. Someone could be sweet at editing but if the content is crappy then nobody is going to make a purchase of your effect any way.