How to:

First you have to read. Not just fantasy, but everything you can get your hands on and ESPECIALLY non-fiction as this helps you achieve consistency.

Second, read

The Masters. Heinlein, Tolkien, and the others mentioned here. Analyze them and their techniques. Read the best and learn from them.

Third, find YOUR vision. A helluva a lot of what passes for "fantasy" are simply knock-offs of "The Quest" motif done by everyone from whoever wrote about the San' Grail to Tolkien to whatever is current today. Try to find something different.

Fourth, well, this is the hard part. You have to actually sit at a computer, a typewriter, or before a bunch of paper with a pen and actually write it down. And rewrite it, and rewrite and rewrite it...ad nauseam.

Fifth, find someone to publist it. There are many books on how to do this.

Sixth, be sure your skin is thick enough to let the inevitable criticisms roll off: "...this is just another copy of the Niven/Pratchett/Tolkien collaberation of 1927..." or "This books sucks ditch water because the author is a twit and Not One Of Us."

Seventh, roll around in the Big Buck$ you earn like $crooge McDuck does...right.