Ok, I have to admit that compassion meditation can be a bit of a downer, though it is something of an acquired taste, so don't give up. Appreciative joy is much, much more fun. The basic idea here is to practice feeling the joy of other people. One nice thing about this is that you can always do this, no matter how badly things are going in your own life. Jimmy Buffet said "It's always 5 o'clock somewhere" and Buddhists would say, "Someone is always happy." When you are feeling down, you really should try this - it hard to meditate on other people's joys without feeling better about your own situation.
We are already disposed to feel happy when others are happy (some of us more than others, of course). Just think about a time when someone near you was truly happy, with a smile that really lit up their face. It's very difficult to look at someone like that and frown or be grumpy. The trick is to learn to appreciate the happiness of others even when it is not blindingly obvious, however. It's easy when someone is grinning ear to ear, but more difficult when they are just quietly contented because our mirror neurons are not firing away behind the scenes. But you need to learn to do this, because people are usually happy relatively quietly.
Just as with compassion meditation, the idea is to appreciate the emotional states of others - their happiness as well as their sadness - so you can empathize with them and treat them appropriately. You can not treat someone who is gravely ill correctly if you do not understand something of what they are going through, but you also can't treat someone who just had their first child properly if you don't understand something of that experience as well.
So the process is very similar to other techniques we have learned so far. We will meditate for 20 minutes, broken into 5 minute segments. We will begin by considering the happiness of family and friends, since this comes easily to us. Then we'll move on to neutral people. Then difficult people. Finally, we will consider the happiness of humanity in general. This last is a bit abstract and sometimes not so easy. It helps to think of the the assignment as being to allow your mind to roam more freely than it normally does - considering not just people you have encountered personally, but people you have never seen before, etc. If you really get good at this, you can expand this last category even further to include not just humans, but all sentient beings (squirrels experience pain and pleasure too, after all).
Now, a few tips: First, you do not have to pick a single person in each category, but try to imagine the face of a particular person at any given moment. Then imagine that face with a big smile on it - the sort of smile that reflects true inner joy. When you get better at this, you can work on appreciating more subtle joys, but at first made it pretty obvious. You can also think of scenarios that generate joy for people, though be careful not to bet bogged down in details, as always. So you might imagine that the person is happy because they just witnessed the birth of their first child, or found out that the person they have loved for years loves them back, etc. Just as its common in compassion meditation to cry, it's common in appreciative joy meditation to smile. So meditate, smile, and enjoy yourself!