Hello Lawra,
Goto WE19, enter your Idoc and execute.
Segments will be displayed, to edit the segment double click on the segment, a pop up screen will be opened there you can change the values of the field.
In case if you want to create a new segment, click on the create segments button or (F5) in the beginning and add your segment and process it by clicking on the Inbound function module.
In the popup L_IDOC_INPUT_WMMBXY give this function module and process it in the foreground.
Regards,
Thanga