Use bamboo canes to mark out the middle posts. Panels usually come 6ft (1.83m) wide, and don’t forget to add the width of the posts into your measurements.
Now dig your holes for the posts. 450mm is a pretty standard depth for timber posts, and more if you have soft ground. If you’re going for concrete posts then you’ll need at least 600mm, but preferably more like 760mm.
Starting with your corners, add 3 or 4 inches of gravel to the bottom of each hole, place your post in and get it straight, square and at the correct height. A post leveller is really handy at this point.