The process around this can variate a lot depending on what system you are coming from and the complexity of your business.
The general best practice i think is to try to avoid bringing forward a lot of historical transaction based data unless it is really necessary for your business. Less is more when it comes to migration.
You should also create a cut-off plan that describe what preparations you should do in the old and what transactions would need to be finalized there.
I would also recommend that you create a pretty detailed migration plan where you document the data entities you will bring forward and describe where you are getting the data from, any manual clean up stages you need and also where these data should end up in your new system.
Performing a complete test migration is also an important step in my mind. And make sure to do that test migration as early as possible so you have time to correct your migration strategy if needed.
When it comes to the technical prosess of getting data into Business Central there are a lot of options available. Personally i like to use the edit in Excel feature as much as possible. But you can also use configuration packages for parts or all of your data. In some situations it also might be a good idea to involve a developer especially if there is a lot of data you need to transform between the new and the old system.
