You need to be in a Kingdom to take advantage of your Kingdom Builder.
You have one Kingdom Builder
You will know your Kingdom Builder is available when you see the Hammer over your Kingdom's Flag. (From your Village, look to the top left of the screen to find your Kingdom's Flag)
After your Kingdom members start an upgrade or a new build, you will be able to visit their Village and apply your Kingdom Builder on their building.
Once you apply your Kingdom Builder to a building, the current build/upgrade time will be cut in half.
Your Kingdom Builder will return to you once your Kingdom Member's build/upgrade has been completed. Please note, your Kingdom Builder will remain in your Kingdom Member's Village for the FULL duration of the build/upgrade. If your Kingdom Member finishes the build/upgrade instantly with Diamonds your Kingdom Builder will not instantly return home until the full duration is completed.