Before you begin the process of haltering your horse, you'll need to prepare the halter. If your horse is acting up, it can be hard to prepare the halter in the moment.  Unbuckle or unsnap the crown piece. This is the strap of the halter that goes behind the horse's ears. It should have a buckle or snap somewhere on it. Have your halter clipped to your lead line ahead of time. You want to be able to halter your horse and immediately lead him where he needs to go. You clip the lead line onto a ring at the bottom of the noseband. The noseband is the portion of a halter that slips around a horse's nose. Call your horse as you approach him. This helps alert your horse to your presence. This way, he's less likely to get spooked by your approach. Call out his name or call out something like, "I'm coming in!" to let him know you're entering his pasture or stall. Make sure you have your halter in your left hand as you approach. You will be haltering the horse from the left side. You want to make sure your horse knows what's happening. If you try to deceive the horse by hiding the halter, he'll learn to be distrustful of your intent. This can lead to a bad relationship between you and the horse. When haltering a horse, you should always approach from the left side. Halters are designed to be buckled on the left. You should approach your horse so you're standing near his head on the left side of his body. You do not want to put the halter on instantly. This can startle your horse. When you are a foot or so away from your horse's head, allow him to sniff your hand. This way, your horse will also familiarize himself with your scent. He'll be able to differentiate your smell from the smell of potential predators in other situations down the road. A nervous horse may benefit from having treats as a reward for staying calm.

Summary:
Prepare your halter. Call the horse. Approach with the halter in your left hand. Walk slowly to the horse's left side. Allow your horse to sniff your hand.