For this meditation, it's good to sit down for a sustained period, for example, around an hour. Sit down, close your eyes and just let your mind wander. Think about whatever comes to your mind. Don't make any effort to encourage certain thoughts or stop others.
Naval Ravikant recommended this technique as a way to deal with unresolved issues that you have on your mind, by allowing them to bubble up to the surface and just observe them.
The reason I love this technique is because I spend most of my waking hours thinking about how amazing my future is going to be, or reminiscing about something that happened in the past, as I'm sure many of you do too. The beauty of this technique is that if you just sit still and think for enough time, at some point you exhaust a lot of these thoughts about the past and the future, and you find yourself actually appreciating and immersing yourself in the present moment.
Source: Naval Ravikant. Where he got it from, I'm not sure ...