Keep it simple. Avoid giving too many steps or trying to determine exactly what needs to happen in the story. Give a simple setting, characteristic, or scenario to start with that can spark the writer's imagination, but avoid a prompt that severely limits how the writer can move forward with the story. For example, give a first line, a setting (an abandoned mall at night, a truck stop the day after a nuclear attack) or a main character (a woman with a bionic arm, a child who can see five minutes into the future). Prompts should be open-ended and have an element of the 'strange' to allow the writer to use their imagination.
Use the local newspaper or even a "weird news" website, such as "Fortean Times" to get ideas for prompts. For example, a news report about strange events such as unusual weather, unexplained behavior or bizarre crimes can work as a prompt in which the writer has to create a story around the event.
Use pictures as prompts. An alternative to a written prompt could be an interesting photograph or object. Try to choose objects or pictures with a strange or unique twist to help aid with the speculative fiction aspect of the writing.