Step one is to sketch a rough draft of the design you want to create for slow stitching. Step two is to cut and place fabric and fibers where you want them in your design. Step 3 is to start layering up individual areas of the design until you achieve the look you want. The next step is to remove each piece as you want to sew it onto your background fabric, leaving the designed piece intact until you're ready to sew them. If the piece is very intricate, take a photo before starting to sew just in case you need to refer to it later. This is the method I use when designing my slow-stitch pieces. It's easy and works beautifully. This method leaves plenty of room for adding or modifying the design as you go along. Consider it a draft that you can embellish or improve as you work.
Broadening horizons in thinking and creative endeavors.