I used an old technique of marking the chequered pattern by initially marking it out with a pencil, but rather than draw it freehand I cut a thin piece of paper and marked it out in 5mm segments.
Prior to working on the lines, I painted the whole barding in the lighter base colour, in this case a light grey.
I drew a line down the back of the neck and body to give me a centre-line to work from.
I then placed one end of the paper on the centre-line to mark out the horizontal and vertical lines. When you get round to the neck area I extended the lines up from the shoulder area and tapered them to the back of the horse's head.
The head was then marked out.
I then used a Micron 005 pen to define the lines and overpainted any errors with the base coat.
Once I was happy with the pattern, I blocked in the alternate squares with black - make sure you start from the centre-line and work out from there to the bottom edge otherwise you can end up with a mismatch when you get to the opposite side.
Then paint in the required colours.
For the shield patterns, I measured the size of the shield and placed it on a gridded piece of paper and used that to mark off the lines.