NHM Writer is the free software that allows you to create your own custom Keyboards and use them. The several itself comes with several built-in keyboard layouts for all the Indic Language. Since the keyboards are all XML files, it is easy to create new keyboards or tweak the existing keyboards
After Installing NHM Writer, by default, it would start automatically with Windows. It can be modified through settings anyway. NHM appears in the Taskbar as a bell shaped Icon.
The keyboard can be activated by using the shortcut keys or clicking on the Icon and the selecting the keyboard.
After, Keyboard selection, the icon changes color, indicating that the Keyboard is active.
You may have to press alt a few times after selecting the keyboard and also deselecting the keyboard for proper input.
Now, you can start inputting text using the selected keyboard in any application. For deselecting the keyboard, press the same short-cut key.
See the official manual here: http://software.nhm.in/sites/default/files/NHMWriter-User%20Manual.pdf