No matter what your craft, there is always the preparation, and getting that right can make such difference to enjoying the rest of your project and having it turn out well. I've been meaning to get this information on this site for some time. When I first encountered sashiko stitching I loved the look, but the problem transferring the design onto dark blue fabric almost made me give it up!
Directions I found back then amounted to actually graphing and drawing the design onto your fabric directly using a ruler and chalk pencil. Not only was that painfully not fun, the design rubbed away before the stitching was done. Iron away white pens seemed a solution, but I still couldn't abide the graphing part.
So I made some full size sashiko patterns and discovered the following method.
Begin with your sashiko design, a permanent ink fine tip pen, some white featherweight fusible interfacing (pellon) (non-woven is best), tape and a ruler.
Tape the pattern to your table. Tape the interfacing glue side (the rough side) down over the pattern. Tape it to the table. Using the ruler and pen trace the design onto the interfacing.
Now lay the interfacing, glue side down, over the back of your fabric and using a medium heat iron, fuse the interfacing to the fabric. (The glue heats, melts and fuses the interfacing to the fabric) Begin in the middle of your design and iron gently toward the edges. I lift and set my iron, rather than sliding it. This helps to keep the interfacing from pulling out of shape.
Now your design is securely on your fabric. It won't rub away as you do the stitching, it will stay easy to see, and it will stabilize the fabric a little without changing how it feels. You will leave the interfacing on the fabric when you are done, there is no need to remove it, it will be covered in your finished project.
One more detail: you will be stitching from the BACK of the fabric. Try it before you decide that you can't do it. It's actually just as easy to stitch on the fabric back, plus you have better control when you are turning corners and crossing over open spaces. Remember to make your long stitches on the finished side tho!
Okay, say you hate method 1 for some reason that I can't imagine (or because you are stitching on a finished garment and can't put interfacing on the back) I think your best method for transferring the design in this case would be to use white sewing carbon. Lay it over your fabric, 'waxy' side down. Position your pattern over it and trace over the sashiko design lines. Check at the beginning to make sure the carbon is showing up on your fabric as you trace.
This method is quick and good for small area, but rubs away if you are stitching a large area.
So here is another option, tho its really an expanding of the second method. After you trace the design onto your fabric using the sewing carbon, trace over it again with an iron away marker. Now it will stay put while you do the stitching.
Since I never transfer by either of these methods I have no pictures!