Cross Domain Migration 2003 to 2012

Hi,

 I've got a 2003 domain with 2 DCs (2003 SP2) which needs to be migrated to another forest. The target forest has 2012 R2 DCs with a 2003 domain and forest functional levels. The forests have DNS and 2 way trust setup between them.

I'm after a step by step guide on how to migrate users, computers and required resources from the 2003 forest to the newer forest with 2012 DCs. Ideally, I'd like to do this with free tools such as ADMT.

Please can someone advise?

Thanks

IT Support/Everything