Start data migration from the legacy business systems to the ERP system as soon as possible.
A real configuration of an ERP system will only happen when there is real data in the system. The mapping of legacy system data to a new ERP is a long difficult process, and choices must be made as to what data gets moved and what gets left behind. The only way to verify what you need to actually run in the new ERP environment is to migrate the data over to the ERP development and test environments and test it. The only way to get a smooth transition to a new ERP is to develop processes as automatic as possible to migrate the data from the old system to the new.
Data loading is not a project that can be done after everything else is ready. Just defining the data in the legacy system is a huge horrible task. Actually mapping it to one of the ERP system schemas is a lesson in pain that must be experienced to be believed.
The scope of a data migration project is usually a fairly large development process with a lot of proprietary code written to extract legacy data, transform and load the data into the ERP system. This process is usually called ETL (extract, transform, load.)
How is data put into the ERP?
There is usually a painfully slow data import facility with most ERP systems. Mashing data into the usually undocumented table schema is also an option, but must be carefully researched. Getting the data out of the legacy systems is usually left to the company buying the ERP. These export - import processes can be complex and slow, sometimes specialized ETL tools can help, sometimes it is easier to use what ever your programmers are familiar with, tools such as C, shell or perl.
An interesting thing to note is that many bugs and quirks of the old systems will be found when the data is mapped and examined. I am always amazed at what data I find in a legacy system, usually the data has no relational integrity , note that it does not have much more integrity once it is placed in an ERP system so accurate and clean data going in helps to create a system that can work.
The Business Analysts (BAs) that are good understand the importance of data migration and have an organized plan to migrate the data, allocate resources, give detailed data maps to the migrators (or help create the maps) and give space estimates to the DBAs. Bad BAs can totally fubar the ERP implementation. If the BAs and management cannot fathom that old data must be mapped to the new system, RUN AWAY. The project will fail.