Idoc triggering

Hi

I am working on an Interface which has to send out material master fields and some fields which are to be obtained by calculations from fields of 3 different tables (keph kekp marm).

Can I activate change pointers to the custome message type I will be creating with basic type as MATMAS05 and custom extension type.

Can anybody explain me the steps for populating the custom fields and triggering the idoc creation, whenever there is a change in the material master data of materials from a particular plant and of particular type.( finished and semi-finished goods).

Thanks and regards

Pratyusha.