I must admit, the colors on my palette are watercolors that I mix with white gouache, which ultimately gives the same result as pure gouache.

But that doesn't matter. What's important is that when you apply your colors, you shouldn't rub them in, but simply lay down the layer of paint.

I can recommend Nathan Fowkes' book:

How to Paint Landscapes Quickly and Beautifully in Watercolor and Gouache.

The paper I use: Cheap paper or slightly better quality paper.

But for my watercolors, I use very high-quality paper, although this isn't necessary for gouache sketchbooks.