I think prior to painting in any style, it is useful to learn how to draw. I found this book recently which I think is really excellent. As you progress through the exercises, the instructor has you work in different media, including paints (but in order to learn how to express a 3d object on 2d material).
The Natural Way to Draw by Nicolaides. It might be found at a library but here is a link to amazon (I don't get any credits for this just for ease of searching)
Otherwise if you're more experimental maybe just study your favorite impressionists and try to copy their style on your own subjects until you find your own way to do it. I personally am into being methodical but this approach (starting realistically) just takes longer.
The Natural Way to Draw by Nicolaides. It might be found at a library but here is a link to amazon (I don't get any credits for this just for ease of searching)
https://www.amazon.com/Natural-Way-Draw-KIMON-NICOLAIDES/dp/...
Otherwise if you're more experimental maybe just study your favorite impressionists and try to copy their style on your own subjects until you find your own way to do it. I personally am into being methodical but this approach (starting realistically) just takes longer.