There is a section on a vid that is on the ilph right weight campaign bit of their website that shows you the correct way to use one.
After I discovered this I find I get more consistant readings when doing it the proper way. It should go to the lowest point of the withers just where it meets the back and round the legs very close to their elbows.
They're not that accurate at all tbh and can also give false readings on mares that have had lots of foals. They are best used to give an indication of which way your horse's weight is going, ie staying about the same, gaining weight or losing weight.