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!
Good luck!!