Do not edit the only copy
Make a working copy before you rename columns or remove empty rows. Keep the original spreadsheet until the club has checked the imported member count and a few representative records. An import is easier to correct when the source is still available.
Prepare one member per row
Use a CSV with one person per row and clear column headings. Include the fields the club actually maintains, such as name, email, phone, member number and notes. Avoid adding a second row for the same person just because two values need updating.
Review before approval
The import review is the decision point. Check the number of ready rows, warnings and possible duplicates. Review a sample at the top, middle and bottom of the file, and resolve ambiguous mappings before approving. Do not treat a suggested match as a fact until someone from the club has checked it.
After the import
Compare the imported count with the club’s source count and open several member records. Keep the source file in the club’s normal secure storage, with a note about the import date and who approved it. The next update should use a fresh working copy rather than repeatedly importing the same file.