Select the cell and highlight the character(s) you want to subscript
Press Ctrl+1
Go to the Font tab
Check Subscript
Click OK
For a single character in a cell, edit the cell first, then highlight only that character and apply Subscript
Use the Formula Bar if you need to select specific characters more precisely
If Subscript is unavailable, use Unicode subscript characters where possible
On Mac, press Command+1, then check Subscript in the Font tab