You can't instantly become a great artist, but you can learn to paint watercolor paintings that make you smile in a few weeks. It's actually much easier for a beginner artist to learn to paint with good habits rather than an intermediate artist change their habits, so that's good news for complete beginners!
I've taught watercolor painting workshops for over 20 years. These are things almost all beginner artists do that mess up their paintings from the start. If your get a good start to a painting, then the rest of the painting is much easier.
If you avoid these 5 mistakes then your paintings will improve dramatically!
1. Not Leaving White Paper at the Beginning
The only part of that instruction I agree with is a big brush (and we all know I prefer rounds.) I agree with painting the entire sky in one go, but leaving holes is critical. If we don't leave holes in our washes we lose the sparkle of white paper.
You can always use white gouache to place white where you happened to lose it, but if you lose the white paper sparkle then you're pretty much limiting yourself to painting a dull lifeless scene. Sometimes this works for night or rainy days. But even on rainy days we want a little bit of sun coming through the clouds and giving life to our work.
You can regain light color by scrubbing out paint or using gouache but you can never regain the effervescent sparkle of pristine white paper.
Always leave at least 1/4 of your paper white on the first wash. Paint all over your painting but leave lots of holes. When in doubt, leave white paper!
2. Too Many Colors
I always tell beginner watercolorists to stick to 3 pigments in a wash before letting the painting dry completely. Advanced artists can get away with more pigments, but the danger point starts at more than 3.
Keep in mind many paint colors are already mixes of several pigments. That's why most paint tubes I use only contain one pigment to start. They're not convenience mixes, which have lovely colors from the beginning - but then you can't mix them with anything before you get mud! I do a lot of color mixing and I'd be limited from the beginning if the paint tube contains 3 pigments.
Use 3 Pigments Maximum in 1 Wash!
If you stick to a limited palette of 12 pigments and only use 3 of those pigments at a time, your paintings will look gorgeously bright and fresh compared to someone else using every color they can get their hands on.
3. Too Much Detail At Beginning
First, too much detail. If you're painting tiny details at the beginning of a painting you're not thinking about the big shapes and value pattern in the painting. You're not thinking big shapes and color temperature pattern.
You're painting each and every stamen on a lovely detailed flower. That flower is usually isolated in the very center of interest, floating in space. All of that detail you've done usually then migrates to the rest of the painting. You've set a high standard for precise details and you want to continue that detail throughout the painting.
Problem #1: When you start with tiny details instead of big shapes your proportions are off.
Even if you're drawn everything beautifully, the shapes of the colors and shadows are just a bit off. If you start big and work toward small shapes you're much more likely to have those proportions right.
Problem #2: If you put a lot of detail in at the beginning of a painting, you tend to put detail everywhere in that painting.This makes the painting extremely fussy. By putting unnecessary detail away from the center of interest you're drawing the viewer's eyes away from where you want them to go. Not good.
If you put the details in very last, you only use as much as absolutely necessary. The simplest way to force yourself into this habit is to use a big brush for 90% of your painting!