They use different interfaces, so it's bit long to explain.
Simple tags are easier to make
The best would be to try to make the same tag from scratch with both version.
Simple tag handlers also include "Tag Files" which are 100%
jsp tags.
Sorry for not answering your question.
You can still refer the following part of the JSP specification:
JSP.7.1.3 Classic Tag Handlers
JSP.7.1.5 Simple Tag Handlers