First of all, you should purchase a top quality rope halter that should be according to the size of your horse. Usually the rope halters come in free sizes but these ropes can be adjusted easily by tightening loosening them according to your requirements.
Now you need to place the rope halter on your horse and make sure it is not too low on the nose of your horse. It should also not be too high which will not give a required grip. So, the rope halter should be just above the nostrils of the horse.
The neck piece of the rope halter loose enough because your horse can pull it over the ears. However, do not make it too much tight which can make your horse uncomfortable. Keep it on average level by inserting your finger between the rope and the skin of the horse which is considered perfect.
After adjusting the rope halter on the nose, you need to make the side knots loose or tight in a way that creates balance on all sides. Make sure that rope halter is not too loose as you will destroy the purpose of rope halter in this way.
Now you need to check whether the rope halter is tied properly which will also help in getting the neck piece of rope halter fit perfectly. Make sure you have checked all sides of rope halter in order to create a perfect balance.
If you think the rope halter is tighter than required, then you should press the knots at the same time in order to loosen them. Make sure that the rope halter is levelled from all sides.