Im not sure if you have had a reply but the steps are as follows, if you have a flex curve preferably 24".
You will also need a large sketch pad preferrably size A2, some chalk or water soluble marker depending on colour of your horse.
Looks like chalk would suit you best if measuring the horse in your picture.
Mark 4" increments along the flex curve.
Have horse standing completely square.
Locate Caudal aspect of scapula - boney landmark to the back of horses shoulder blade.
Measure approx 2"behind this point and mark with water soluble marker or chalk.
This is reference point 1.
Starting at this point mark lines on the horse at 4" increments
Number them 1 through 5.
If you run your fingers from the last rib of the horse up to the spine that is reference point 5 or T18.
Then start at point 1 and place the flex curve across the horses back from near side to off side make sure that the flex is bent into the shape across the horses back.
Carefully take the flex curve place it on your sketch pad and draw a pencil line from near side to off side on your paper using the under side of the flex that was against the horses back to give you the shape .
Mark this as your tree point on the paper.
Take the rest of the measurements through to point 5 and mark these on the same sheet with point 2 above point 1 and point 3 above point 2 and so on.
Shape the flex curve to the shape of the horses topline.
Then under the 1st measurement draw the horses top line.
On the sheet mark on the top line the reference point for point 1 so that this can be corresponded in the measurements of the rest of the horse.
On the top line tracing also mark where the horses spinal process T18 should be.
You can take this to your saddle maker and see if they can cut that shape out for you.
I hope that this helps you.
If you have any further questions just please ask.
All the best with your fitting.