I use a technique where I play a scene out in my head. Always the same scene, always the same outcomes and the same process.
For example “Walking down a beach, see a small shell, pick it up, turn it over and notice the interesting pattern, put in pocket, go to the sea shell stored a few feet down the beach waving at a people, sell the shell, take the money and buy a small rock statue, take the statue home and place it on the window sill… etc”
The trick is make it memorable and not specifically related to your own life so you can’t get side tracked subconsciously (“Oh no! I forgot to buy sea shells!!”). I find a narrative works well, and the whole thing tells a story.
The way to get started is when you are EXHAUSTED and ready to fall asleep anyway, and to repeat the same scene/steps every night from that point on. Eventually the series of images and events will tie to “sleep” in your mind and I rarely get past the first few parts of the sequence.
Essentially counting sheep! same idea really. After a while you may get bored of one story and make up another. I’ve gone though a half dozen over the years I guess.