Okay so because it's taken an inordinate amount of time and medical professionals for me to actually get an answer, I'll spill the beans on what mindfulness is.
Mindfulness is being more present doing the task at hand than it is being all up in your head about the task.
People might shorthand it by saying it's being in the present" or "being aware of your body," but that's not all of it. Sometimes you're planning a vacation for the future or comparing two products. Those aren't really being present in your own body right here right now. But you'd be mindful if you were focused and working on the task of planning versus flustering over which places you wanted to go and getting more stuck and thought-holed than actually doing the thing. It would be mindful to compare products, and not mindful to get into a tizzy over the details of which is better.
Mindfulness doesn't mean you only have pleasant emotions. It means you're better able to cope with emotions without getting stuck in your head about it. Sometimes you might get overwhelmed by an emotion, and focusing on your body and how it exists in space can be good for grounding. What muscles are tight where, focusing on relaxing them, what textures or temperatures you feel. Letting the emotion pass through you, acknowledging that emotion exists while you focus on your body until the emotion is manageable.
As for how you do it, that's personal trial and error and practice, but I found it so hard to get a straight answer as to what mindfulness actually was, the goal I was trying to do, that I figured now I'd actually got a definition I'd share.