Most pre-stretched canvases are pre-primed these days, meaning you can paint straight onto them but often times the canvas lacks tooth and absorbency and paint will then soak unevenly into the weave. (Tooth affects how well the paint binds to the painting surface). Artists often give the canvas another coat or two of gesso which will prepare (or prime) the surface for painting, making the surface slightly textured and ready to accept paint.
1) Use a large brush, laying brushstrokes down in one direction. Add a very small amount of water to the gesso if it is too thick to spread evenly
2) Use a separate dry brush in a crosshatch pattern to smooth out any fine lines that form (otherwise these lines will be visible through your painting)
3) Lightly sandpaper between each DRY layer.
4) Repeat steps 1-4 until the desired surface texture is reached. Wipe dry canvas with a cloth before beginning your masterpiece