Open an image or begin a new document;

Activate the Quickmask using the left-bottom button in the image window. If a selection is present the mask is initialized with the content of the selection;

Choose any drawing tool. Paint on the Quick Mask using black color to remove selected areas and white color to add selected areas. You can use grey colors to get partially selected areas.

You can also use selection tools and fill these selections with the Bucket Fill tool. This does not destroy the Qmask selections!

Toggle off the Quickmask using the left-bottom button in the image window: the selection will be displayed with its marching ants.