Select the cell and highlight the specific character(s) you want to format
Press Ctrl+1 to open the Format Cells dialog
Go to the Font tab
Check the Subscript box
Click OK
Use subscript in a formula bar only if the text is entered as part of a cell value, not within a formula result
For Unicode subscript characters, insert them directly into the cell if available
On Mac, use Command+1 to open Format Cells, then choose Subscript in the Font tab