This design looks complex, but broken down to a step at a time, it is easy enough to do.
There are a few things it helps to know before you start stitching. The first is that the printed marks will wash away completely when you are finished stitching, so don't get the cloth damp before that! Also, please don't iron it before it is washed or the marks will be set and won't wash out :-(
Next, you are meant to stitch through two layers of fabric, so start by folding the unprinted half of the fabric to the back of the printed half. You can baste the edges together to keep the layers from moving apart as you do the sashiko stitching.
One more thing. Prepare your sashiko thread by opening the skein into a long loop and cut through one end of the loop. This will give you 36" lengths, which are a good length for the sashiko and weaving designs. (I cut through both ends and stitch with 28" lengths for regular sashiko stitching - longer is better for sashiko and weaving) I like to loosely braid the skein and pull the threads from it one at a time - no tangling!
Start this design by simply stitching the diagonal marks in one direction.
You can see in the following photos, that I like to start threads with a tiny knot which I stitch into the margin of the design. I use these fabrics to sew other projects, or I back and hem them, or I put borders on them.
Sashiko stitch the diagonal markings in the other direction to create X's
Start the blue thread. There are tiny dots marked along the edges to mark your start and stop your stitching. I start the weaving thread a line over from the corner because I found it harder to figure out the corner. It is easy to go back and do it after there is a line woven next to it.
Bring your needle to the front of the fabric at a small dot.
Weave the thread under the X's and push your needle back through the fabric at the next dot.
Bring your needle back to the front very close to where you put it through to the back.
Weave back down the row.
Notice that I have turned the needle around and am using the threaded eye end of the needle to pass under the threads.
Move the he next row bu taking a stitch form the dot at the end of the row you are weaving and coming back tot he surface at the next dot.
Note: At this point you can use one sashiko thread to stitch several rows because they are short. later the rows will be too long. I don't like to end a thread in the middle of a row so I plan to always start and end at an edge. It means you use a little more thread.
Here is what the back looks like:
Continue until all the rows are stitched.
Next we start the last step, which is actually the same as the last one but stitching on the opposite diagonal. I changed thread color to make it easier to see. Might have been better to pick a darker color :-)
Start at the end of a blue line.
Still using the eye end of your needle, weave the line passing under the X's.
The threads are loose at this stage, and they should be, so resist the urge to want to make them all snug. It will tighten up as more lines are woven.
Here are samples of the design stitched in two colors and three colors. the blue and yellow one has been finished and washed, so the threads are even and snug.
.... but thats just how I like to do it. Other people stitch wonderful pieces with parallel lines evenly spaced. It's a matter of what you enjoy plus what is functional for your project. The right thing to do is to suit yourself!