Open up your icon or program "steam client"...once you are on its (opening) home page...
at the very top of that page should be a menu bar where you can view your games...click on the games tab...and view games library...then another screen/window will open...and near the top of that page should be a menu bar with the word TOOLS (3rd menu bar down from very top) once that screen/page opens there should be a black screen with a list of downloads...click on the Sid Meier's Civ V SDK. If all goes well it will be on your desktop after it is downloaded. Click on the SDK icon then on the World Builder ICON and the wait a minute seems like eternity...now there should be a pic of the world/earth with a big hammer over it and 3 buttons, new map, load map
and mods buttons.