validation in sales order

hi experts,

i am entirely new to user-exits , i know validations in sales order can be done in the program MV45AFZB with

userexit_check_vbak

userexit_check_vbap

userexit_check_vbkd

userexit_check_vbep

userexit_check_vbsn, i dont know how to do it,

can any one write me how to do it by talkin a simple vallidation,

how to implement it in cmod ie abt applyin in the transaction.

plz i need this , i can help u in givin u points.

thnx already prakash.