user exit for me51n

Hi Friends..

I need to create an user exit for the transaction me51n. Here when i click the save button i need to call one pop up.. in that pop up when i press the yes button i need to call the transaction md04.Here i need to pass the input for Md04. When i press the BACK button i need to come back to the same screen in Me51n.

For this requirement i found one function module exit for the transaction me51n.That function module exit contains around 8 no of function modules...

I would like to know that how to proceed to do this user exit... Where should i write the codings for the existing function module exit or whether i need to create a new function module?

Help me regarding this.. Since i am new for doing User exit..

Regards,

GowriShankar.