You can just paint on the mask layer. Make sure just the mask layer is selected by clicking on the mask layer thumbnail (see below), choose a Paint Brush and either black paint (to add to the mask and erase the image) or white paint (to remove the mask and reveal the image) and paint on the mask layer. As you do, you can see the effect on the image.
You can also paint in grey for a semi transparent mask.
If you right click on the mask, there are also some Edit options
Edit Mask displays the mask as black and white.
Refine Mask loads in into the Refine Selection panel.
Release Mask just moves it outside the image layer. So it is no longer nested.
P.S. If you are using Affinity Designer, you will need to be in Pixel Persona to paint on it.
It sounds as though you're working on an image layer, not a pixel layer. Image layers are treated as single objects, so you can only perform global operations such as rotating the image or adding layer effects: you can't work at the pixel level (e.g. for copying a selection). Go to the Layers panel, right-click the layer and choose 'Rasterize...' so that the designation changes from '(Image)' to '(Pixel)'.
