Where wild minds come to rest
Okay, first off, I just want to say why meditation will help your MD. Meditation is an exercise in focus, training yourself to choose what things you pay attention to. In short, it won't make you stop having those "trigger" thoughts that usually send you into a big 'ol daydream. What it will do is help you notice those thoughts and let them peacefully float away without spending three hours developing a new plot line or character.
Side note: meditation does a lot more than just that. I'm attaching an infographic of stuff that meditation can do for you.
Find a quiet, comfortable spot where you can sit without being disturbed. I don't recommend lying down, because it's really easy to fall asleep, but you can do that if you like. If you're going to do the classic legs-crossed meditation pose, be sure to sit on a cushion or something so that your butt is above your knees, otherwise it will really start to hurt after a while.
When you're comfortable, close your eyes.
The basic premise of meditation is you want to pick something to focus all of your attention on, something that isn't very stimulating. The three "anchors" that I learned are the following:
-Breath: don't alter your breathing in any way. Just try to notice it. What the air feels like moving through your nostrils, down your throat, into your lungs. It's all about awareness. Try not to use any words, even in your head. The goal is to be non-verbal.
-Body: what does your body feel like? Are there places that are stiff that you didn't notice until now? How do your clothes feel on your skin? What is the sensation of your butt on the cushion or the chair? Feel the weight of your hands resting on your legs or at your sides. Et cetera. Again, keep in non-verbal.
-Sound: you should be in a quiet place, but nowhere is silent. Listen to and absorb the tiny sounds around you. And yet again, this should be a non-verbal experience. Try not to label the sounds you hear by thinking "that's the air conditioning" or "birds chirping outside", simply let the sounds wash over you and think of nothing else.
To clarify, you are picking ONE of these three anchors, and focusing only on that. And let me tell you, it will not be easy at first. You'll sit there and pay attention to your breath, or your body, or sound, and suddenly you realize you've let your mind wander for most of the time.
Here's a good analogy for the way you deal with your wandering mind during meditation:
If you've ever had a puppy, you know that they are kinda wild. You have to train them. And if you get a puppy and you tell it to stay, it probably won't stay. It will wander off. But is it fair of you to get mad at the puppy? No. It doesn't know any better. You haven't trained it yet. Your brain is the same way. When you sit down to meditate, you will tell your brain to stay and it will not. So what do you do with this disobedient puppy? You don't get mad, you don't scold, you simply bring it back, and say "stay". Again, it will not stay. But if you do this enough times, the puppy begins to learn that when you say "stay", you don't allow it to wander off. Eventually, you will say "stay" and the puppy will know what you mean. So it is with your brain. Every time you find that your mind has wandered, take a second to notice where it has gone, and gently bring it back. Thoughts will come and go, but the goal is to notice, but not pay attention to them. Keep your focus on your anchor.
If you have a really hard time with this (which is actually really common, considering the constant stimulation we experience in modern society, and especially considering that you probably spend hours every day letting your mind wander all over the place) you can try listening to guided meditations as a warm-up. You still need to focus, but it's easier when you're listening to someone talk.
There's a free app called Insight Timer which I highly recommend. It has TONS of free guided meditations of varying lengths. When you get the hang of that, you can move on to using the app to time your unguided meditations.
Start your meditations at five minutes or so (set a timer so you don't have to keep breaking focus to check a clock). It will still feel like a really long time. But after a while, when you get better at focusing, it starts to feel not so long. When you really get in the zone, you totally lose track of time. You can gradually work your way up to longer times, just like adding more weight to an exercise machine as you build strength.
With practice, this should improve your ability to ignore MD thoughts when they come to mind, allowing you to manage the amount of time you spend daydreaming.
I hope this helps!