Start with the God Mythos that you want to use. Is it one that exists already, or is it your own, or a combination of the two? This then will give you the greatest culture upon your planet.
Now draw a map of your new planet, with the continents, land masses, and oceans named. Now decide how many cultures are on your planet, and then place them. Also create one unique culture that you and others won't recognize where you got it from.
Now how magical of a world do you want? How many suns and moons do you want?
Don't do the earth, reason being that it's too restricting on what you can and can't do, for history has all readyalready done this tofor you!
Now you are on your way and the rest will flow from you. Remember the old books you read; use them as examples to stay away from. You can use a small percentage from foreign works — just have your own ideas so outnumber the borrowed ones that it shows. It shows you liked that concept and thus why you included it; but don't do this to often, otherwise it looks like all you know how to do is borrow from others. (The book Dinotopia did this too much, and that cheapened it.)
Also create a list of rules alterations to give to the players from the start.
Now have fun and create.