Give all worksheets unique names, and remove blank worksheets.
Add text to cell A1.Screen readers start reading any worksheet from cell A1.
Use a simple table structure for data only, and specify column header information.
Include alt text with all visuals.
Use sufficient contrast for text and background colors. Do not use color alone to convey information.
Check your work with the accessibility checker.
Microsoft: Make your Excel Spreadsheets Accessible to People with Disabilities
Microsoft Training Video: Create Accessible Tables in Excel
WebAIM: Microsoft Excel, Maxmiizing Spreadsheet Accessibility