How do I enter Unreal Editor for 4.12.3?
After install, click to the library tab in the left side of launcher. Or just use the orange “Launch” tab in the top left corner. Wait a few seconds. Then it will open the project window. Select the directory of your project, then type in a name of your project and press “Create Project”. And again wait a few seconds. And it’s ready. Now you are in the Unreal editor.
https://docs.unrealengine.com/latest/images/Engine/Basics/Projects/Browser/NewProject_FPSExample.jpg