Sit or lie down and relax. To start, think of one good thing you did yesterday. It doesn't have to be anything big. Maybe you smiled at a stranger, or maybe you were getting annoyed at a family member but managed to let go of your anger. Now try to think of two more things.
If you can't think of anything - think about the fact that you've taken the time out to sit and do this exercise, or at the very least, read this passage. You're trying something new and trying to improve yourself. That's a big deal!
Next, think of someone who has helped you recently, and think about the good within them. Take time to appreciate the things they have done for you.
After that, think of a friend. Think about some of the memories you have had with that person, and appreciate the good within them.
Think of someone who is going through a difficult period in their lives. Think of the times that they have helped others, or shown strength while going through hard times.
Finally, think of someone you have difficulty with. See if you can focus on the good parts about them, or find some good in things they have done. If you can't, remind yourself that like everyone else, they want to be happy.
To close out this meditation, spend some time thinking that everyone wants to be happy just like you. Your friends, your benefactors, the person that's giving you trouble. Repeat this phrase a few times to finish:
All beings want to be happy. May they be happy.
Source: Real Happiness - Sharon Salzberg