Lakshmi is of the goddesses whose mantras are relatively easy to learn and which tend to give rapid results. There are many different ways to use Lakshmi mantras, but one of the best is the repetition called japa.
All you need to do is sit comfortably with your eyes open or closed, as you prefer, and repeat the mantra. Generally one would do this in multiples of 108 while counting on a mala, but others prefer to repeat the mantra for a certain period of time. The repetition can be out loud, whispered, or silently. Again, let your own preference be your guide.
Consistency is important. While any level of repetition has its benefit, establishing a pattern of say, 108 times every day, and maintaining that over a traditional 40 day period bring the greatest benefit. But most important is getting started. Even if your goal is only 10 consecutive days, accomplishing that goal will bring substantial benefit.
Here are some suggested mantras:
For overall spiritual growth use the Lakshmi Gayatri Mantra:
OM Maha Deyvicha Vidmahay
Vishnu Patnicha Deemahee
Thanno Lakshmi Prachodayat
For financial benefit:
OM Shreem Maha Lakshmiyei Namaha
For greater happiness:
OM Shreem Shree-aee Namaha
For overall abundance:
OM Hreem Shreem Kleem
Maha Lakshmi Namaha
Here’s a brief recording of the mantras so you can hear proper pronunciation.