Where can I find instructions on how to do this?
https://slicer.readthedocs.io/en/latest/user_guide/module_segmenteditor.html
and
https://www.slicer.org/wiki/Documentation/Nightly/Training#Slicer4_Image_Segmentation
Specifically, for the task @ljod mentioned, you can use the scissors tool to quickly clear all of the unwanted label region with a cut in one slice. For example:
