After your initial Dynamics 365 for Finance and Operations environment is configured, it is time to perform data migration from the legacy system. The following diagram depicts the data migration process that can be used in your implementation project:

  

As shown in the preceding diagram, data migration activities start from identifying data entities in Dynamics 365 for Finance and Operations and the corresponding data in your legacy system. Once the data elements are identified, you need to work on mapping these data elements. Data templates in Finance and Operations can be used to define the mapping rules and default values. Often, the data in a legacy system is not in a very clean state and you do not want to bring unnecessary data to your new system. If possible, clean or filter such data in the legacy system.

For large data migration scenarios, you may need custom or ETL tools for data extraction and data cleansing. You can use these tools to create data files and data packages as needed for Dynamics 365 for Finance and Operations. Once the data packages are prepared, you can load and validate the data in Dynamics 365 for Finance and Operations, Enterprise edition.

Data migration is usually not a one-time process; it needs to be repeated multiple times to get to a clean state. Repeat the process multiple times in the development environment and every other environment, such as training, testing, and UAT, to catch any errors.  

Remember that for production to go live, data migration activities usually need to be performed during system downtime. Apart from a successful run, another aspect that you may have to manage is the total time to run the data migration activities. This time must be measured and optimized to fit your overall downtime window.