To set your input device as StageSound in Zoom ensure StageSound is opened and is turned on. Then in a meeting, click on the small arrow next to the Mute button at the bottom left corner of the window. A menu will appear and under "Select a Microphone" choose StageSound.
When you have finished using StageSound, follow the above steps again to access this menu, this time selecting your usual microphone.
If you wish to improve your sound further, in that same menu, at the bottom, now click "Audio Settings..." This will then open a new settings window.
At the bottom of this window, several tick boxes should be seen. If "Enable stereo" is shown make sure that box is ticked.
Then click on the "Advanced" button at the bottom right corner to show more settings.
Make sure the box next to "Show in-meeting option to "Enable Original Sound" from microphone" is ticked.
Then close this window by clicking on the red button in the top left corner. This will now return you to the standard Zoom meeting view.
You will now see a message in the top left corner of the window saying "Turn on Original Sound". Click the small arrow to the right of that message and then select StageSound.
This will mean that in future every time you select StageSound as the microphone, Zoom will default to the highest sound quality settings possible.