Determine Custom Fields

If a marker in the database doesn't correspond to a built-in field in FLEx, make a note in the spreadsheet that it will need a custom field.

Decide on a good name for the field you will make.

Be sure you have filled in the columns for what writing system(s) this field needs, and what level in the hierarchy it will occur at.

At a later step, you will create all these custom fields in the empty database, and then make a backup before doing the import.

You may do that iteratively--add some custom fields, make a backup of the empty database with the newly added custom fields, attempt an import, and then realize you need still more custom fields, and repeat the process.  That is a normal part of an import process.