To build a canvas, you first choose the size and thickness of your stretcher bars, stretch the canvas around them (you need strong hands for this!) and staple, then prime and sand and prime and sand the coats of primer as many times as it takes to get you to the background texture desired (usually about three rounds of this, in my case). Or, you can go to any art supply store and buy a ready-to-use canvas (which I do use from time to time, if one crosses my path). I wouldn't expect most people to pay attention to the direction of the brush-strokes of the primed surface under the painting, but somebody should. A blank canvas isn't blank at all.
PS - If you look closely, you can see my handprint in the lower left corner on many of these works- my way of showing that I built the painting from surface up.
