Hi Manyo, I used a template which shows the dimensions of the banner for each device, as you have found out, it is different on different devices. A quick search of 'youtube banner template' and under pictures, the first one is the one that I used.
The central area of the template is the area that is shown on every device and so needs to be where all of the information is.
As for software to use, I followed a youtube guide using pixlr.com which got me the banner I am currently using.
Hope that helps.
Yakkiran.