I have not seen any drawings showing how the visor opens and closes but there is a picture in Achtung Panzer no.2 Pz III on page 22 that shows the drivers position from the inside on a Pz III G and it looks like there is a handle on the left side to work the visor.
Here are a few pictures showing the visor from the outside in the open position.

[/img]

Another view of the early Pz III

[/img]

If you would need any pictures or measurements of the Stug III G let me know.