When writing a story/fanfic my first intention is to think of a good plot that suits or mixes well with the characters, once it does, the main source of a good scene is to make it interesting and at the same time, make sure it makes sense. It's also essential to take things slow, make a meaning and not rush things to the main bit.
I don't often write my ideas out, most of the time, I can'T remember. But seeing as i have my own journal, I'll try XD