Which is the best Way to Build & Design Buttons, Icons for Mobile application of Android and IOS platform.
I'd say coming up with several designs that follow basic GUI guidelines and testing them on the devices is a good start. I also think trying to keep them in one continuous style helps giving an overall complete look. Keep in mind they have to work in varying resolutions. So either find a good balance between complexity and simplicity or make different versions for different resolutions.
For both systems there is a lot of documentation on this too:
Other than that, I am not exactly sure what "best way" you are asking for.
Actually depends on your ability. If you work with Adobe Photoshop or CorelDraw With a general understanding of software, you can be a Icon Designer. Just in about IOS maybe It may be a little hard, because The operating system has limitations. for example XCODE in MAC has some simple Icons and you can use them. In generally I think Adobe Photoshop is a very useful design program for all of code writers and programmer.