You know how people make collages,
by very carefully cutting out pieces of magazines?

This allows them to put a house on a woodland background,
and then a person wearing a suit in the foreground, and then a face on top of that person, from somewhere else.

They are splicing a scene together,
it is a kind of art.

Digitally clipping collages is a very useful technique for assembling your scenes,
when you assemble a collage that covers all the features of your painting...

You can simply focus on painting,
as you have now assembled everything in it.

Creating a digital collage prior to painting gives you three big things,
a glimpse of your finished work, shape reference for everything in it, and of course colors.

If you download a squirrel from the internet,
or better yet chase one around your neighborhood until you get a good photo...

Your photo becomes a hundred times more useful,
if you get rid of the background and just keep the squirrel.

As a bonus, if you have a scurry of squirrels, you can easily arranged them,
because they are cut out for you, and the backgrounds don't get in your way.

There are two ways to cut something out of a digital photo,
using the lasso tool, or my means of masking.

If you chop something off with the lasso tool,
it is gone forever.

But with masking, it is just using black to hide something, and white to keep it on the screen,
so if you make a mistake and cut something off with black, just put some white in there to get it back.

Search for GIMP Layer Mask Tutorial, or Krita Transparency Mask Tutorial, or whatever program you are using,
find a video; as it only takes a minute to learn.

You usually have to add a mask, or a transparency mask,
and then switch to it, and pay attention that you are painting on the mask not the image.

And as a huge bonus, you are not limited to black or white,
you can also use gray, this can be very useful, for fluffy-puffy tails for example.

In closing, when using a collage as a digital under-painting,
you never get lost, and you can fix all the things that don't seem to fit quite right.

Combined with layer management where pants, shirt, hair are always on unique layers,
you get to make final touches that only impact specific objects in your painting.

Using digital photos from sites that have liberal license like Pexels or Unsplash,
can quickly get you started, and make even your most complex works less challenging, and more and beautiful.