So when you notice your mind wandering this is mindfulness. Imagine you're coding all day long, and then you have this moment where you notice that you're not paying attention to coding, but instead thinking about what it would be like to be a ninja. You might have been thinking about being a ninja for 5-10 minutes before you realized it. But this moment when you realize, oh hey I was thinking about something other that what I thought I was thinking about is a moment of mindfulness. And this is what you're practicing.
So when you notice your mind wandering, be happy, this is a moment of mindfulness. Your mind was probably wandering for a while before you realized it, but this moment is the moment you realized it.
https://shivayashiva.wordpress.com/2014/01/12/centering-prac...
There is also a kind of meditation practice called "just note gone" where the idea is to start giving absense of stimulation a distinct notice (by noticing when something you had been perceiving is no longer there). I think most sit down and meditate practices tend to focus on that kind of thing (feeling the absense of stimulation as a distinct thing) via different methods.
A common form of meditation is focusing on your breath. This meditation is composed of two parts, trying to pay attention to breath, and noticing when your mind wanders.
Most people think the most important portion of that exercise is focusing on breath. But in truth it's the second part, the noticing.. The first part is growing your attention, but the second part is practicing your mindfulness.
From there, maybe experiment with silent meditation. I got there by focusing on successively simple stimuli (city -> picture -> mental picture -> simple shapes -> dot -> nothing).
It, quite literally, is practice.
I don't use an app, but I know a few people that have had some success with them.
Best decision I've made this year
You might want to check out the book, The Power of Now by Eckhart Tolle. The free Headspace meditations are also a good lightweight introduction IMO.
Follow your breath, count each breath from 1-10, repeat.
Start over if you lose count.
Continue for 2-200 minutes.
You’re meditating.
If you need more help try the Urban Dharma podcast, Kusala is a great guy.