The basic idea here is to work on equanimity. Equanimity means basically evenness of mind. In this particular case, we are seeking to reveal the fundamental similarity of all people so that we can think of everyone the same way - at least from a moral point of view. Like everything else, this is a simple concept to grasp intellectually, but it requires meditative practice to really make this a part of the way you live your life.
Take yourself, a friend, a neutral person and a difficult person. For the first 5 minute period, put yourself on stage and the friend next to you. Focus on their faces side by side as you meditate. Compare yourself to the friend, then the neutral person, then the difficult person in turn. Then imagine all four people holding hands in a circle as you compare them. If time allows, start back at the beginning with yourself and your friend. in the second 5 minute period, your friend is on stage and will be compared to you, the neutral person and the difficult person in turn. By the end of the 20 minutes, you will have compared each of these people to all the others many times.
The details of what you think are, as always, not that important. But since this is the most abstract kind of meditation we practice, I will give you a basic schema similar to what I use myself most of the time.
Imagine the faces side by side on stage
I seek pleasure and avoid wish to pain.
Friend seeks pleasure and wishes to avoid pain
As with one, so too with the other
Then I imagine offering both people some particular pleasure (with no strings attached) and they both readily agree
Then I imagine offering them both some pain and they both reject it
They are the same
One interesting thing is that when you offer someone a million dollars or a poke in the eye with a sharp stick, not only will they make the predictable response, but they often smile and laugh when they do this. It's as if their choice is so obvious it's amusing to even be asked. But that is the whole point - it should be obvious that these people are fundamentally similar, yet we need to work to keep this in mind because we are so used to thinking not of the similiarities, but of the differences between people.
Now for some theory:
we tend to focus on differences
interesting psychological studies of how this happens and how it can cause problems - students with t shirts
essential vs accidental properties
the properties we do not share are not essential and those we do share are essential
therefore everyone is fundamentally the same, regardless of what separates us
as we said before, it's critical to work on keeping this truth alive, because the first step in treating others badly is always to view them as other than yourself (zimbardo)