If using a fabric canvas, make sure that the fabric itself doesn't have too much give. That also applies for how tightly wound the canvas should be.If the canvas wasn't stretched enough around the base, it will retract whenever applying the brush. This will only make for a frustrating experience for the artist and affect the paint application.
An optional suply when prepping your canvas is using PVA. This glue, when applied, will stiffen the fabric canvas. This is also an effective way to further stiffen your canvas after stretching. Some artists would still prefer not to use it so that the texture of the cotton or linen is still present when painting.
What is used to prep the canvas is gesso. Acrylic painters have the option to not use this. They could just use acrylic paint for prep. Oil painters, however, have to consider that oil paint can rot the wood, or acrylic paint, underneath it. So for painting studies this really isn't a necessary process If this will be a painting meant for preservation, though, it is best to go through precautions so that the time spent will not be of waste.
To apply the gesso evenly, a foam brush is likely the most effective. This provides an even, thin coat. It is in the artist's preference how many layers of gesso is applied, but 2 coats is sufficient.You can also choose to tone the gesso. Just apply some acrylic or oil paint of a color of your choosing. Dark painting compositions may require a dark grey colored gesso, while as an earthy- colored painting is complimented by using burnt-umber. Depending on how smooth the artist wants their canvas, sand each layer of gesso, or, just sand the last layer.