Thank you for the question. I also learnt along trying to figure it out.
Here is how I approached it :
Click the Stack icon on your Text Formatting toolbar (as seen in the picture)
Choose the stack type, I chose the ones as shown.
Enter the text on top right corner of the window.
Choose subscript size in the drop down list below. And voila
Subscript.png 21.5 KB
Thanks, that is exactly what I was looking for.