To begin the meditation, find yourself a comfortable position where you will not be disturbed while you meditate. Ultimately you should be sitting in the full lotus position if you are able. However, I have read teachings from Hindu masters that indicate a half lotus is perfectly acceptable if the full lotus position makes you too uncomfortable (especially for Westerners who did not grow up in the lotus position). If for some reason you cannot be in the half lotus position, you may even perform the meditation seated with your spine comfortably straight. Meditating while lying down works too, though you may tend to fall asleep.
The most important aspect of your body position during mantra meditation, is to have the spine straight so that the energy can flow easily through your chakras without being blocked. That is one of the reasons why the full lotus is so often used, it brings your body naturally into a good alignment.
Take a deep breath and let your body relax. Then begin to repeat the mantra out loud or in your mind, whatever is comfortable for you. It is recommended to voice it out loud because you are creating a vibration of energy which resonates out into the Universe. Repeating the mantra in your mind also produces the same vibration, just with your thoughts instead of your voice. I have had good success repeating the mantra in my mind, and in my opinion, the vibrations of thought can be just as powerful as the vibrations of your voice.
As you repeat the mantra, om shrim maha laxmi yei swaha, see Lakshmi in your mind. Visualize her gold coins of prosperity showering you in your life. Concentrate your awareness on Lakshmi, sometimes you might imagine her smiling as she bestows good fortune upon you. Let your breath flow naturally, as your mind calms down, your breathing will slow, this is natural.
Say the mantra 108 times with Love and Compassion in your heart. 108 is a very sacred number in Hindu tradition, all sanskrit mantras should be repeated 108 times for a complete meditation cycle. If you only have a short amount of time to meditate, 12 times is a good number, but 108 is the best. Even saying it 15 or 25 or 68 times is perfectly fine, sometimes the real world requires your attention or you may not be able to finish your complete meditation for some reason. Consider 108 repetitions as a meditative practice goal.
It is nearly impossible to keep count while meditating, that is why prayer beads known as a Mala are used. You hold the mala necklace which has 108 beads and each time you say the mantra you move one more bead through your fingers until you get to the beginning of the mala again. It works incredibly well, freeing your mind to concentrate on meditating.
As you meditate, you may find that your mind wanders onto other thoughts, gently move your mind back to Laxmi, or the mantra, or just nothingness. I have found it especially effective to just focus all of your attention on just saying the mantra. Try to find the time to perform the meditation about once per day for maximum benefit. When you really start to get good at it, mantra meditation can actually be addictive. Especially at the higher spiritual levels of consciousness, such as Yogi Masters attain, when they begin to experience bliss and unity. Seek balance, it is not necessary to spend *all* of your time in meditation.