Since the IATF 16949 is rather complex standard, the transition process should be conducted as a project. First step should be to conduct a GAP analysis to determine to what level your existing QMS is compliant with IATF 16949 and what needs to be done to achieve full compliance with the standard. Once you determine the gaps, yo can develop project plan for the transition, defining what needs to be done, who will do it, what resources are needed and what are the deadlines.
Then you can start implementing new requirements of the standard. The most important changes are related to context of the organization and addressing risks and opportunities but almost every clause has suffered changes to some extent.