This is more traditional meditation; mindfulness is something which is also practiced in a sort of everyday life, but here's a 5-minute guide to meditating.
So the first thing you need that's often sadly overlooked is a special place which you make for meditation. This can be as simple as "I am going to set an exercise mat in the middle of the carpet in my living room." Keep the space to be relatively clean. Cleaning is itself a great way to dedicate your mind to wholesomeness and order.
Once you've cleaned a space and imbued it with symbolic power, "here is where I am going to meditate", you sit there. Sit however you like the first several times. There is a special way to sit involving a "Zafu" (a pillow that when compressed is still 4-6 inches wide) that you may want to research, where you sit on the edge of it with much weight resting on your knees, and your feet are in a position that neither restricts blood flow nor pinches nerves. The cushion angles your hips forwards, which keeps your back straight. But if you don't have the disposable income to get the cusion for this, just sit down however feels comfortable. Place your hands wherever feels most natural for you to be able to breathe deeply.
Once you have a comfortable seat in that place, it helps to have a little speech to focus your attention and remind yourself what you are doing and why. One Buddhist pre-meditation speech I heard says (with a bit of misremembering and translation error doubtless thrown in): "I am here to end my confusion and distraction. If ending confusion is more important to me right now than everything else, I will succeed. If it is not more important than something, I won't. I am not here to learn some new ideology, but just to pay attention to my own mind: I want to awaken to the causes of my distraction so that I can learn how they work and restrict their power over me."
Now, have something repetitive that you can come constantly back to. Distraction works like alcohol works: if you ingest a little, it subtly compromises your mind in just the sort of way that makes it more likely to ingest more. So this is why the space is clean and you are sitting with good posture without pain, so that you remove the most common distractions of mess, sleep, discomfort.
Many people use the rhythm of deep breaths as the repetitive thing they can come back to. If you are a more visual person, you might want to instead put an object of religious veneration, be it a cross or a Buddha or what-have-you. Someone whose peacefulness you can study is good.
I once helped develop a more "active" picture for a friend who had grown up with either ADD or ADHD, I don't remember which. She said she needed something more like a wholesome movie. We tried to structure it so that it could still be repetitive. Then a week later neither of us could remember what we had done the week before, so we created a new one but as a mnemonic we structured it around 5 elements. It looked something like this:
Dwell on each of these for 10-15 minutes as a way to eliminate
distractions. If you find yourself thinking about something else,
allow yourself to think about it for a little, then return to the
element you're on.
1. (Air) You are walking through a forest, and the wind is blowing
through the trees, blowing through your hair, removing any
impurity or distraction that is an obstacle to your growth. You
settle on a direction where these winds of change are at your
back, letting them push you wherever they need you.
2. (Fire) There is the smell of smoke, then a feeling of heat
behind you, then light, and you find yourself running, running
through this forest which lights up, needing to take swift,
dynamic action to dodge the flames and keep ahead of the blaze,
to get to safety.
3. (Water) The fire opens up into a clearing with a river. There are
some scared animals: a fox, some bunnies, a squirrel. You are
holding them to your chest to reassure them, and then you lay on
your back in the river, rafting them safely downstream. Your
compassion brings a calm, soothing rain which starts to fall,
extinguishing the blaze into steam.
4. (Earth) You get up and set the animals free as the river bends
near a rock face, and walk along the side of the hill. You find a
trail that goes up it. You feel these smooth, ancient, unmoving
stones beside you as you ascend this cliff. You feel calm and
relaxed with these stones that never need to be anything other
than what they are.
5. (Void) The trail opens up to a landing. You sit down on the
landing with this solid rock beneath you and look up at the night
sky. You see the valley in front of you, even more mountain behind
you, and are struck by how big this place is and how small we are.
The bottom line is that anything works, if it's not too complicated and allows you to say "hey, I am thinking too much about this other person who insulted me the other day, I'm just going to come back to this thing I'm supposed to be focusing on and focus on that instead."