Open an image or begin a new document.

Activate QuickMask mode 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 QuickMask with black to remove selected areas, and paint with white to add selected areas. Use grey colors to partially select areas.

You can also use selection tools and fill these selections with the Bucket Fill tool; this does not destroy the QuickMask selections!

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