As with so many things in painting, there is no right or wrong when it comes to how much detail you put into the initial pencil sketch you do on a canvas. You don't even have to use a pencil; many artists use a thin brush and fluid paint. Put as much or little detail into your initial sketch as you wish. Personally, I think it's ultimately better to do less, to remember that a painting isn't simply a colored-in drawing.
Once you start adding paint to your canvas, you're going to see less and less of your drawing or sketch. Trying to retain your sketch as you paint is a recipe for frustration and stiffness. The initial sketch is a starting point only; a few guidelines for the overall composition which soon disappear under the paint. You don't need it for long as the colors and tones of paint you put does become the guidelines for the next bit of painting.
I typically do a very minimal sketch on the canvas, as the photo shows. I'll have thought about it, visualized it, and probably run my fingers over the canvas as I decide on the final composition. Then I take a pencil and very lightly sketch in the main lines of the composition. I've darkened the pencil in the photo so it shows up more; in real life you can't see the pencil unless you're at arm's length from the canvas.
The sketch done, I then block in the main shapes and colors with paint. This replaces my pencil sketch as the guide for where things are in my composition. For a more detailed an example of this, take a look at this step-by-step demo where I first block in the blue, and then block in the other colors.
In other paintings, if I have a very strong image in my mind of what I want it to be, I may combine blocking in with mixing colors directly on the canvas. There's an example of this on the next page...