What is the best way to chant it?
I think that:
Chanting that mantra is a characteristic of a form of Buddhism (a school, a tradition of Buddhism) called "Nichiren" (see Nichiren).
There aren't many users on this site who know about Nichiren, so it might be difficult to answer your question.
The most widespread Nichiren-oriented society outside Japan is called SGI
(Soka Gakkai International) -- I expect you can find more from their web site and/or by finding a group of SGI people near you.
The core Buddhist practice of SGI members is chanting Nam-myoho-renge-kyo and reciting portions of the Lotus Sutra (referred to as gongyo), and sharing the teachings of Buddhism with others in order to help them overcome their problems.
The practice of chanting Nam-myoho-renge-kyo was established by Nichiren (1222–82), a reformist Buddhist monk who identified the Lotus Sutra as the core teaching of Shakyamuni Buddha.
Applying the Teachings
Practice is supported by faith and study. SGI members study the teachings of Nichiren in order to deepen their understanding of the principles of Buddhism and the processes of inner transformation. Study strengthens faith and conviction, which finds expression in practice.
Faith, in Nichiren Buddhism, is grounded in the experience of applying Buddhism and seeing improvements in the quality of one’s life. Faith could be described as the ongoing effort to orient one’s heart toward the ideal of Buddhahood—the continual unfolding of one’s inherent potential for good, the ability to transform any negative circumstance into a source of growth and benefit, and a life dedicated toward helping others do the same.
Their web site in India seems to be https://www.bharatsokagakkai.org/
My experience (in Canada) was that they are happy to have you join them their group, to meet to chant with them, and to explain their doctrines to you.
How can I have my desires fulfilled by that mantra?
I'd like to (but I'm not sure I can) explain that, in a way that's simple and accurate -- perhaps someone else will, who knows more about that form/school of Buddhism than I do.
I suppose it's easier to practice than it is to teach.
Or you may find an answer by reading the web pages I referenced -- see also for example Buddhist Concepts.
I'm referencing SGI's web site, because I think they're the biggest organisation of practitioners outside Japan whose practice depends on the mantra that you asked about.
There are other forms or schools of Buddhism too, which don't use that mantra. It's said there's a unity, that all schools of Buddhism have a lot in common with each other, perhaps it is all relevant to the "Buddhist method of worship".
I also recommend these topics, on this site:
I suppose that it's important to hear, understand, and practice Buddhist doctrine (i.e. "dhamma" or "dharma"), possibly with other Buddhists -- and that mantra is connected with that, part of that.