This is the process: You get a prompt. You set a timer and start writing right away, without thinking about what you want to write, without stopping. Write for 5 min about whatever that prompt brings to mind.
If I were to follow this suggestion I won't write anything at all. And it is not because I don't have writing experience or never been published. I did and I was.
The problem is that I come to writing in a very different way. First I search the situation where the prompt is an absolutely crucial part of the narrative. Then I start to wrap around it the cabbage leaves of explanatory matter. And only then the first and the last sentences appear.
Nevertheless, this is my "first sentence post"
https://steemit.com/freewrite/@mgaft1/weekend-freewrite-part-1-the-first-sentence