How to use Customer Exit / Enhancement MM06E003 at ME21N ( Document Number Range )

Hi all,
This post is a tutorial for using MM06E003 ( Document Number Range ) exit at ME21N (or other Transaction).

I hope this can help others out.

I will skip 'How to find user exit' in this post, if you need more information, can refer to this thread at SCN.
'List of Exits / Enhancements  for ME21N', refer to this thread at SCN.

1 => We need to create a project in CMOD. Give it a name and a short text.