This example sets the text for shape two on slide one in the active presentation and then makes the second character a subscript character with a 20-percent offset.
Dim charRange As TextRange
With Application.ActivePresentation.Slides(1).Shapes(2)
Set charRange = .TextFrame.TextRange.InsertBefore("H2O")
charRange.Characters(2).Font.BaselineOffset = -0.2
End With
This example formats every subscript character in shape two on slide one as bold.
With Application.ActivePresentation.Slides(1).Shapes(2) _
.TextFrame.TextRange
For i = 1 To .Characters.Count
With .Characters(i).Font
If .Subscript Then .Bold = True
End With
Next
End With