Disclaimer: I am not a horse trainer or a professional, I just have experience halter breaking horses as well as training my beast :P
A bit about horse behavior
First of all, horses are herd animals. This means that they have an alpha, or a "leader." When you own a horse, you need to be their leader. This means you control their every move, you make them move and they don't make you move. You should be able to tell them to canter without running towards them while lunging. You need to be able to make them back by wiggling the lead rope. You need to be able to move their hind end away from you with just a look down their side. Basically, you need to be in control of the situation.
First things first
In order to have your horse listen to you, invest in a rope halter. They work really well because they will apply pressure in sensitive areas when needed. If you think your horse might have a harder time listening to you, purchase a rope halter with lots of knots across the nose band, like this one. When you need to get your horse's attention, this will work really well! I also suggest purchasing a longer lead rope with a leather tassle on the end. This will really help when you have to...get after your horse when he is behaving badly.
After you have these items, practice the idea of poll pressure. This can be accomplished many ways. For me, the easiest way for my mare to understand pressure release was to tie her to a rail using my rope halter. As soon as she pulls back, there will be pressure on her poll. Once she feels that and decides to step forward, the poll pressure leaves. This concept is really important because your horse will need to understand that they can release poll pressure by coming forward, and basically, listening to you instead of resisting.
Now, onto the good stuff
Next, you will need to pretend, tell, and imagine yourself as the alpha. Since horses are herd animals, you need to present yourself as the pack leader. This is very easy to do! Just always drive from their shoulder. For instance, when you're walking them, always be at their shoulder and not at their head. When lunging, be at their shoulder and not at their end. Pack leaders are always at the shoulder as this is the driving part of the horse. If you are consistent with this, your horse will understand that you are the driving force.
Speaking of driving them, in order to gain their respect and show that you control them, you will have to run them. I know it sounds a little excessive, but using a method like join-up or just plain lunging until their tired will show that you control the situation. If they are being naughty, work them until they are out of breath. The reward for them is to not work and to breathe and relax next to you. This shows them that you are now in charge.
Once you feel like you have their respect, we can now start ground work!
Backing, Directions, Gaits...the list goes on!
When walking, your horse should always be with you. She should not be in front, she should not be behind, and you should not feel like she is controlling the situation. She needs to be out of your personal space and listening to your every command. The best way, for me at least, to accomplish this is to take steps, stop, then back up if she goes into your personal bubble.
For instance, I will walk my mare a couple strides, stop, and make sure she stops with me. If she keeps walking while I'm stopped, I immediately back her up! You can do this many ways but I usually will start teaching horses to back up by clucking, moving my arms in a pumping fashion against my sides while wiggling the rope, and walking towards her. This will make them back up. You don't need to make a huge deal out of backing, just do it for a couple of steps so that your horse knows they cannot move into your space. Eventually, your horse should be able to back up with you simple wiggling and not moving your feet.
Another exercise you can do is to make them do a little trot circle around you. This is where the longer lead rope comes in handy! By telling them to move out, stop, and face you when you stop means that you are controlling them. Simply begin by standing at their shoulder and making them move away from you in a circle. Make sure to stand at their shoulder and not drive from their hind! Trot them for about a rotation, then make them stop, switch directions, and move on. When they stop, you need to have them facing you with both their eyes, no butt allowed! This exercise teaches them to move away from you on your command, as well as listen to what you're saying exactly.
Basically, whenever you are doing anything with horses, you want them to never point their hind ends at you. If they do this, that means they are ready to kick. You are much safer with their face towards you and hind end away. If you horse stops with their face not 100% facing you, get them to move their hind end by gesturing the lead rope towards their hind. This will make them move it away from you.
All of these exercises help with control. Ground manners is simply about respect and control. If you have these exercises under your belt, you have a respectful horse and you are ready to take on the world!