Here's an outline to work with when considering how to create sound effects, when to use them and where to use them in a script to be most effective. Sound conveys meaning. Sound stimulates visual images in our minds. Radio is a hot medium, that is, the listener's imagination and experience are involved in giving the story depth, substance and meaning. Sound effects can be used for such things as setting and place, conveying action, solving certain narrative problems, and evoking characterizations.
Real sounds are more convincing than synthesized ones.
But most things do not make the sound we think they make. A sound effect is like a mini-drama, with a beginning, middle, and end. It is meant to indicate some action or event, and it should follow through to complete that action. A sound effect most often consists of more than one part - usually several parts. A door opening isn't just one click of the latch.