One of the advantages of language learning is that it gives you access to literature that wasn't available to you before. For example, I learned a lot from reading Ken Morita's book 誰でもたちまち絵がうまくなる「トレース水彩画」入門 (Anyone can quickly become a good painter, "Trace Watercolour Painting" introduction) on Kindle unlimited.
If you can't read Japanese, check out his Youtube videos. They are mostly self-explanatory. Let's try and follow Morita's procedure.
First, you trace a picture into watercolour paper. It's important that you use quality tracing paper. I tried with cheap tracing paper and it didn't work well. If you can afford it I suggest buying Pilot's carbon plastic paper. The watercolour paper should have no texture, the smoother the better.
Next, use black ink and water to paint it in grayscale. Painting in grayscale first makes the process easier because you can focus on the lights and shades without having to think about the colours at the same time. It's important that the ink you use becomes waterproof when it dries. I suggest Pilot's ink or any other ink brand used by comic artists.
Once the grayscale layer dries, start applying the watercolours. Begin with light colours, then apply darker colours, add a shade colour (e.g. purple) and finish with a white poster colour (gouche) for the highlights. I suggest using a watercolour pan set (I use the Petit color 48 pan set) instead of tubes, it's much easier. Once the colour layer dries stretch the paper by thoroughly soaking it with tap water (don't worry, the watercolours will not go away with the water) and using a set of binder clips. Note that you can do this before painting as well by using gummed watercolour paper tape.
And there you go, this is the result. Probably I should have simplified the lines and chosen brighter colours, but I felt this method really made watercolour painting easier and faster. I didn't talk about more advanced techniques such as masking. I encourage you to watch Morita's videos and explore them by yourself.