Hi Saroja,
The best introduction is to read the beginning of the Ukelele Manual (PDF), which you find inside Ukelele through the Help menu.
There’s lots of detailed stuff in that manual, but section 3, Quick Start, is short and gives lots of important information.
The main thing to understand with Ukelele is that it’s not the application that changes your keyboard behavior.
In MacOS, the journey starting with your keypress and ending with a character appearing on the screen is going through many parts of the system software.
One of those parts is called the keyboard layout and this can be replaced, either by another one from Apple’s own built-in collection of keyboard layouts, or by one that you have customized yourself with Ukelele.
So all Ukelele does is: it lets you customize existing keyboard layouts (and install them, too) in a safe and intuitive way.
After installing, you can activate those customized home-made keyboard layouts just like you do with the ones provided with MacOS.
Happy Ukeleling :)