If you want to use an existing IDOC og segemnt as template, you can seach for existing IDOCs and segmenst using transaction WE32.
Developing IDOC segments
WE31 Developing IDOC segments. Custom
developed segmensts must start with Z. Do not use underscores !
After the segment definition has been created, go 1 screen back
and release it Menu Edit-> Set release. From this screen you
can also check the segment menu Segment -> Check
Creating basic IDOC type
Transaction WE30