Although these are simple principles, our society is too complex and out level of development is too primitive for us to be able to obey them. It's like saying we should try to live without ever doing anything wrong. We just can't do it. If it was necessary to try to obey these principles to be moral, we would all be extremely immoral. There are about 6 billion people on earth, and many times that number of sentient creatures. Most of these people and creatures are trying to make their lives better by either taking advantage of, hurting, or at the least ignoring almost all other creatures. There may be a very few people who can try at all times to help all other humans and sentient creatures, but they would have to live extremely simple lives. Almost all of us live lives that are complicated enough that we can barely understand the effect we have on the few people closest to us, much less understand the effect we are having on all the other people in the world.
Each time we try to keep these principles in mind, humanity evolves a little bit. We add a little bit to the amount of positive morality in the world. If we talk to other people about these ideas, that effects same to try to do this also. My favorite way of testing whether something is moral is to ask what would happen if everyone did it. If everyone tried to keep these principles in mind, they would then be thinking about and discussing the ramifications of actions in terms of these principles. This would often lead to solutions to problems that made life better for most people and most animals.
Cognitive Dissonance is when you have different ideas or beliefs that conflict with each other. This is a painful situation for human beings, and most of us do whatever we can to get out of this situation. We do this by changing one of our ideas or beliefs so it no longer conflicts with the other ones. For example, if you believed that it was wrong to kill people, and you were a soldier in a war and killing people, you could most easily relieve this cognitive dissonance be redefining your enemy as "not really people." Cognitive dissonance is why it is so hard to keep these moral principles in mind. We know what is most moral, and we want to think we are moral. At the same time, we know we are not actually obeying these principles. There is a conflict between the way we act and the ideal way we would act if possible. The whole point of this moral system is to keep this conflict in mind, which means learning to tolerate this cognitive dissonance. This is the first step toward the eventual evolution of a true morality by humanity.
What is a good democratically elected representative of the people? For example, what is a good Senator who represents a state? To the people of the state, he might be considered good and successful if he gets a lot of money for projects in the state, for example, new roads. These projects in general make the people's lives in the state better in many ways. However, for every dollar that Senator gets for his state, there is one dollar less for other states, and people's lives in the other states are made a little worse. Therefore, a politician who is very successful at getting money for his state is considered a bad politician by people in other states, and the money he gets for his state is often referred to derogatorily as "pork barrel" money. So, the people in his state think this Senator is acting in a moral way, by trying to help his state. The people in the other states think he is acting immoraly. We end up with all the Senators competing with each other to take money away from each other and give it to their own states. To act morally, instead each Senator would try to keep in mind what was best for (at least), all the people in the country. To act morally, voters would try to keep in mind that a moral vote would be for the Senator who would try to help the whole country, not just the Senator's state. Now, we get to this "just keep it in mind" part. Suppose there was a someone running for office who promised to not try to take care of the state, but to take care of the whole country. It wouldn't work to vote for that person, since all the other senator's were looking out for their own state, and this "moral" senator would be completely ineffective at helping his or her state. However, if we keep in mind what the ideal situation would be, and talk about it with each other, maybe eventually we will evolve to the point that any Senator who persued the interest of his or her own state at the sake of other states would be considered immoral by everyone, even the people in the Senator's state, and a person like this would never be elected.
If you eat a hamburger, a cow had to be killed. Obviously killing the cow did not increase its happiness or fulfillment. To absolutely obey this principle, you would not eat the flesh of any sentient creature. There are people are able to live their lives this way, but most of us can't. However, what all of us can do is keep in mind that, when we eat meat, an animal suffered for this to happen. We can also try to keep in mind ways of eating meat that would cause the least suffering possible. For example, we can try to support companies that raise animals for food in the most humane ways possible, such as free range chickens and humanely raised cows. Some people who eat meat try to eat only parts of large animals, because of the Buddhist philosophy of "one body, one soul." In other words, fewer animals large animals have to be killed to feed people than small animals. So, to follow UM principles, you might eat meat because it makes you so happy, try to keep in mind that other people are making a living by raising animals for meat, try to keep in mind the ways that raising animals for meat might be injuring other people (such as parts of the rain forests being destroyed to create places to raise cows), try to keep in mind the suffering of the animals, and try to keep in mind ways harm to other people and animals can be lessened, and their happiness can be increased. This doesn't mean that you have to dwell on this and therefore can't enjoy the meat. It just means to spend a little time trying to keep these ideas in your mind. You might even think about how things could be in a perfect world. This is a great way to try to apply UM principles. You ask yourself "what would this situation be like in a perfect world?" My idea is that eventually we will be able to produce a meat substitute that tastes just as good as meat, is better for us, but doesn't involve killing any animals. Sort of like tofu, but much better.