Like anything else, it just takes practice. In this one, the sky is done with oil paint, and needs to dry before the pencil work can begin. Not counting the drying time, it can be done in about 30 minutes.
Amazing you can even do the sky in 30 mins given the coverage of the trees' greenery and all the bits in between the leaves. Must require a very fine touch or is it possible to overlap the blue a little and then remove it from the greenery by some kind of very fine oil "eraser"Well done.
You can overlap. In this case, the sky color (oil paint) did go over the fronds, and part of the trunks. I use typewriter erasers to remove it from the trunks, but the frond color (pencil) goes over the paint, once it dries. You just have to pick your pencil color considering the change the background blue will have on the pencil color.