How-To: Bulk import users with custom fields?

We are migrating from another system and need to import about 5,000 users. Each user has several custom fields that we need to preserve, such as 'Account ID' and 'Subscription Plan.' I've seen the basic CSV import functionality, but the documentation is a bit sparse on how to map columns in the CSV to our custom user fields. What does the header row of the CSV need to look like for the system to recognize these custom fields? Is there a specific format required for the data in those columns? I want to make sure I get this right on the first try to avoid creating a mess. If anyone has a sample CSV file or a checklist for a large-scale user import with custom data, I would be extremely grateful.