Last week we made backgrounds. Mine was just with plastic wrap and some salt, using 3 colors: lemon yellow, magenta, and cobalt blue. When it was dry, I drew on the leaves from my reference photo above.
In the picture below, you can see on the right, where I have begun to paint negatively around the leaves that are my main focus. I am trying to keep my color clean and fresh at this point, and only enough of a value change that I can see my picture clearly, and my leaves come into focus. I generally wet an area, apply a color, in this case cobalt, and drop in other colors such as magenta or yellow. I am not trying for a flat, uniform color at this point.
(I did use masking fluid to mask out some of the flowers, since they were so small, I was afraid I'd lose them. I also used a pen nib to mask tiny lines to the leaves.)
After this first round, I drew in some other details--a few more leaves, some stems, etc.
You tend to lose some of the details as you paint, so I generally add them after each time I paint a value.
Below I'm beginning my second value, still wetting a section, then dropping in color, as long
as I can tell a value change. I also painted the purple centers of the flowers.
Here is the painting after my second round of negative painting.
Remember that negative painting is painting what is NOT the positive image.
So, in this painting, part of a leaf underneath a leaf could be part of the negative
space. So here you can see that I've painting shadows behind some of the leaves.
(I made it dark near the edge, then faded the color out)
(I usually save this step for the end, but I didn't want to forget to mention it)
With a very dark, I am painting behind the stems and also creating a hexagon pattern
that I hope will serve to unify the painting.
Valentine painting for a stained glass look.
I've drawn this valentine pattern onto a paper that I've done a background texture on. (This particular texture was done by drizzling and spattering masking fluid, drying it, and then adding color wet into wet.

Since my pattern is quite busy and colorful, I chose a dark purple to fill in the spaces between the hearts.
Here is the finished version. If you want to create a negative space around
the outside of the heart, you can wet the area and drop in color. Or you can choose to simply outline it.
For those who want to do the trees, here is the tutorial from Louise DeMasi that I was talking about.
I hope that helps you. She's very good at explaining. I did mine long before she posted this, but it's pretty helpful.