step by step procedure for doing LOOK UPs

Hi,

i want call lookups from my message mapping.. originally i'm getting data from flat file.. i want to load this data into R/3 system by using RFC ..

( FILE - XI -(RFC) -R/3) so, but i want to connect anothor R/3 system through the look ups..

for example.. in my file doesn't contain VAT NUMBER.. that contain in another R/3 system.. so, now i want to collect those VATNumber also alsong with the File details.. so, for this how can i do..total how many systems i need to configure and how many communication channels we need to create.... and what type of user defined function can i create..

I'm using graphical mapping.. plz give me the possible steps.. i gone thru the blogs also but i did't understood..

thanks

babu