Sit down, assume a cross legged position. Don't slouch or lean to one side. Hands on your knees or clasped in front of you.
Close your eyes, slow your breathing, and try to count to 10 with your breath. Restart the count if you lose track of what breath you were on or think about anything other than your breath.
So: in, out (1). in, out (2). in, out (3). And so on until you reach 10.
If you breathe slowly and try to really focus on your breath and the count, this will be more of a challenge than it seems.
This is essentially the practise of zazen.
Also, when a thought pops into your head, don't try to push it out; instead, just let it go. One of the major benefits of meditating frequently is that you're able to observe your own mind in action. Instead of say, reacting to something that makes you upset, you might be able to detach yourself from the situation for a moment and make a rational decision about the best course of action. This is mindfullness.
Also, keep in mind that meditating once a month is essentially useless for your overall wellbeing. It is a practise, and as such you need to put time into it every day if you want to improve and experience the benefits of meditation. Some zen masters even believed that the sitting practise of meditation (zazen) was enlightenment in itself. The idea being that the practise and the goal are one and the same.
Another way to do it that I like if I'm feeling lazy or haven't meditated in a while, is to use a simple mantra:
In, out, calm, relax. You say "in" and "calm" while inhaling, and "out" and "relax" while exhaling. Its a simple way to calm your mind if you find the 10 count too difficult or get fidgety.
Also, nasal breathing is key. Always try to keep your focus on your breath.