If possible, the first time you do this, rather than reading ahead, read each stage and follow its instructions before scrolling down to the next stage. (I hesitate to use the word instruction, because writers can be a bolshie lot who take instruction poorly, but in this case just do as you’re told as constraints are good for you.)
Give yourself about twenty minutes.
1. Have a timer, notebook and pen at the ready, and a favourite book at hand (probably a print book but could work with ebook).
2. Close your eyes. Open the book at a random page. Put your finger on the page. Open your eyes. See what word your finger has landed on.
3. Take the first letter of that word. Set the timer for three minutes and write/brainstorm a list of as many words that you can think of that start with that letter of the alphabet.
4. At the three-minute bell, stop.
5. Look over your list quickly and circle your two favourite words. Trust your instinct here.
6. Now close your eyes and use your pen to stab your piece of writing: whichever word you land on is your third word.
7. Set your timer for fifteen minutes, then take these three words to generate a flash fiction or scene or the start of a short story or first chapter.
Adapt this as necessary, but again, stick with the exercise and its constraints for a while if you can.
(And yes, I post this on a Wednesday, but it was a Friday when we first did it at the Festival of Writing 2016. And someone did generate a really good one and email it to me by first thing Monday morning.)