Simple, easy...
Create new layer, fill with white. Helps to name your layers of course.. For burn I use "Multiply" and I change the "Layer Mode" (Green Arrow) to Multiply as well. I paint my darkened areas using like 10% opacity in black.
Create another layer, fill with black. This is our "Color Dodge", changing the layer mode as well and then I can paint using white, set it about the same opacity.
Gimp version.
Same exact trick.. Red Arrow..!
Paint.net version
Pink Arrow, brings up the layer properties. Here we can control multiply & color dodge.
Simple safe dodge & burn, very handy for making jeans along with various other shading you may want to safely place away and use again and again and again...!
As you change each layers properties or mode the white & black layers appear opaque.
