You just practice letting thoughts come without having an emotional reaction to them.
This is the key at first, and why it’s better to limit it to 20 minutes or maybe 30 minutes at first. We’re so conditioned to having an emotional reaction to our thoughts, that it is a little exhausting and intimidating to allow yourself to have the attitude of letting them come out of their cage, letting them have free reign, if you will. That’s the irony or perhaps paradox of meditation: it’s not about stopping your thoughts, it’s about setting them free. And then the practice is: to observe them without having an emotional reaction to them. And that takes some work, especially at first, because in many cases, some more so than others, our thoughts are intimately intwined or tied to an emotion. With some thoughts it almost feels like our emotion is inseparable from them. But with practice even the most “nuclear force like bound” emotion-thought duo will gradually loosen, and that in itself releases a lot of negative energy stored up in your system. And that’s a place from which a lot more options become available (“Doors start opening in places we never even knew doors existed”) and lot more happiness has the chance to get in.