also see the faq entry how to compile vkdt such that you can use your pentablet as pressure-sensitive input:
you need a specific version of glfw to support it.
you can for instance clone https://github.com/hanatos/glfw,
for instance to /home/you/vc/glfw, and then put the
following in your custom bin/config.mk:
VKDT_GLFW_CFLAGS=-I/home/you/vc/glfw/include/
VKDT_GLFW_LDFLAGS=/home/you/vc/glfw/build/src/libglfw3.a
VKDT_USE_PENTABLET=1
export VKDT_USE_PENTABLET
of course you can also just use your mouse to draw masks.
for a description how to draw and how the strokes are adapted to the image,
see the draw module documentation.
to quickly add a draw module and the wiring around it to your image graph,
insert the draw block (e.g. by pressing ctrl-b in darkroom mode and select
the draw block, then insert it before a certain module in the graph).
drawn masks are especially useful in conjunction with retouching and inpainting. it can also be required to not only change the exposure of a masked region, but also grade it differently to match the surroundings when removing a shadow, for instance.
also see the (simpler) graduated density module which can create a simple linear gradient mask without brush strokes.
transform modules (e.g. lens, crop, frame) interact with the draw module
because they change the size and position of the image on the screen and thus
render drawn masks inconsistent. the best way to deal with this is to place the
module using the drawn mask before any transforming module (if possible). this
way, you can change the transform later on without invalidating the mask.
while drawing the mask, you can temporarily disable any transform modules which
come later in the pipeline (usually further up in the list in the right panel)
by clicking the circle symbol on the module expander in the pipeline config
tab.