So I touched on the basics of manners and respect in leading and grooming. These are the most important building blocks to training a horse. If they are overlooked, certain holes in the horse's later training will appear, and will be harder to deal with.
Lets talk a bit about respect. Respect is an understanding by the horse that you are in charge, and great things can come from you, but also that you have rules and boundaries that need to be obeyed before there can be affection and praise for a job well done. Every owner should decide what level of respect they want, but there are some basics I think everyone agrees on. The horse should respect your space and not: kick, bite or step on you, whether on purpose or accident. The horse should pay attention to you and your requests and he should generally look for reward and approval from you.
There are some gray areas that vary person to person, and you should determine the level you are okay with and realize the horse will most likely test you every once in a while and take it a step further than you like. You must be willing to stick to your own rules and keep him within the boundaries you set. Some examples of the debatable behaviors: asking/begging for treats or attention when he could be patiently standing still, looking at other things rather than where he is going when you are riding or leading, pulling the lead rope/reins out of your hands, and moving after having been asked not to move.
My personal guidelines are that he should remain doing whatever I have asked him to do until I ask him to do other wise. That is... If I say stand still, you should stand still until I ask you to go. If I ask you to go, you should go until I ask you to do something else. Very simple. I also hold the belief that since they can make mistakes, I allow one mistake before reprimand. That means on the horse's bad day he may make the same mistake twice, as a way of pushing the boundaries. I have accepted that and will correct it. The important part here is not that he is PUNISHED, but that he is corrected and shown what is the right choice to make.
If the horse can understand your body language and you can clearly communicate with him for leading -forward, turn, stop, back- and he can accept your touch all over for grooming and you can move his body away from you if needed, you are probably ready for lunging. If you have not done your "Homework" as described, lunging is going to be a very, very difficult and possibly damaging process.