I'm far from an expert, but here's what I've picked up:
The most important technique for watercolor is to work light to dark. Unlike other types of paint, you can't just paint a lighter color over a dark patch. (You can lift pigment after, but it typically leaves slight discoloration, and you risk damaging the paper.) Instead, with watercolor you are building up layers of translucent tints to create your image.
With that in mind, let's consider the types of areas that you listed for a generic watercolor painting: wash, fill, shadows, highlights, and details.
Mark out your highlights, because you want to work light-to-dark. You don't want to put pigment on top of an area that is meant to be light.
Washes - typically lighter and less saturated than the subject - come next, so that you can paint the details over top of it. You can do a background wash later on, if you're careful not to disturb the other parts. However, it's certainly easier to do first.
Fill in the main subject, taking care with your highlights from earlier.
Add details and any dark shadows. I'm listing these together, because the exact order will probably depend on the piece. I've done both ways: sometimes details first so they were also shaded, sometimes details last so they'd be sharper. (Or details, shade, touch up details..)
If you have a dark wash but light fill/details in your painting, you could instead try:
Paint the light areas, wait to dry, then carefully outline and do the wash.
Mark the light areas, paint the wash around them. Clean up edges by lifting the pigment, then paint the light areas.
Your frog painting is a great example for discussing this, since it has a dark background and dark spots on the lighter subject. My approach would be something like:
- Paint the frog limbs and body color (minus black spots). This would include the color variations e.g. at its nose, after laying down the base color.
- Leaf base color
- Lines on leaf
- Dark spots on frog: outline with a fine brush around the edge and the highlight spots, then fill in
- Dark background section at top edge (this could be any time after 2 though, really)
- Shadows: probably on frog body first, then on the leaf
Of course, this is art - everyone has a different preference, experiment with what works best for you!