You always want to position your easel where there will be enough light hitting your canvas. Ideally, the light that hits your canvas should be even and unobstructed. Make sure that the light is not so direct that it causes glare. Also, do your best to ensure that there is room behind your canvas so that you can step back and get a good look at it from a distance. In other words, avoid placing your canvas somewhere with a wall directly behind it.