Jeannee He doesn't mention it so you can choose to either put on the green and brown directly on the canvas (harder work) or have a VERY little of liquid clear on it first and then apply those two colours. It does make it easier and it is how I did it.
Another option would be what Sunnylady does a lot and that is to thin the paint (the green and brown) before applying it. One can do that with adding a little paint thinner and linseed oil to the paint (making it more fluid), then put your brush into it and put it on the canvas. It will slide smoothly!