How do you spell the name John in Chinese?

There is no "spelling" in Chinese - it is written with characters. Chinese names consist of two characters (usually), chosen for their meaning and sound, not from any preset list as in the west. One could transpose the sounds of the name John into Chinese characters, but that would be confusing at best to Chinese people. The name of the Apostle John in the Bible (which is where the name comes from) is written in the Chinese Bible as 约翰 yue1han4 (pinyin romanization), pronounced somewhat like the German Johann.