To keep myself from ever becoming inundated when creating worlds, I tend to build a ton at once. This allows me to switch between focused construction and casual inspiration whenever I want, and often allows me to accidentally come up with an idea that works perfectly for a world while working on another.
Of course, this does mean that most of my worlds are stuck in development, but I don't really mind.
When considering a world as an individual, however, my focus varies. In a sense, how I go about building the world is as important in defining the world as the pieces I put in it.
For larger projects, worlds that I hope to someday use for a series of novels, I use a top-down method. I figure out the genre, the sub-genres, and maybe some themes, and then I create systems that reflect those themes. For example, in my "low-fantasy" world, one of the prevalent themes of the world's history was that magic was becoming increasingly obscure and mythical in nature. As such, I focused first on building a magic system that would incorporate that, then the various ages as defined by how societies would handle magic. Both as I created the magic system and the ages, specific stories throughout the world's history formed, and I began expanding on the characters and plots for those. In these larger projects, I create a world for the purpose of writing the stories within.
For smaller projects, I work largely in the reverse. I create interesting and particularly varied characters—oftentimes characters I created in games like LittleBigPlanet—mix them all together, and then split them into potential groups. For each group, I create a world that accounts for their particularities. For example, one group included an amazonian-styled warrior of the jungle who fought with large crystals protruding from her gloves, an adventurous explorer/conquistador, and a painter who used paint for combat. To make them all fit together, I created an imperial setting where the jungles were being colonized, and a magic system where all water is replaced with a magical substance—that can also be found in crystallized form. From there, I figure out what sort of tech and societies might develop, and from that, I decide on a genre. While these worlds also allow me to create stories, their purpose is instead simply as an avenue of worldbuilding. Practicing with them help me with the bigger projects.
That being said, those are only two general and simplified methods. Because the method I use to build a world is very closely related to which world I'm building, specific focuses vary wildly—for some, magic systems come first; for others, characters, or plots; for others, races, creatures, and technology. And I'm always jumping between worlds.