Using a ruler to mark the entry points and my trusty diagram for the general layout, I use my trusty Sharpie to mark the roads on the boards.
Back to the sharp knives! After drawing the roads, I trade in the pen for a knife and carefully cut just to the outside of my marker lines.
You want to be sure you cut all the way through the mat. Don't worry if you cut into the foam. We're going to be adding texture to these roads that will hide any small cuts. Press down on the part of the grass mat that's supposed to stay on the board and carefully peel away a corner of the section to be removed. You will find if you were careful about cutting through the mat, you can lift off a section despite the glue.
The next step is to blend the roads and riverbanks back into the landscape and get rid of the precision line where they meet the grass areas. Not only will this step give your roads a nice base for adding texture, it will also seal the edges of the grass mats and hide any gaps between the wood battens and foam at the exposed edges of the roads and rivers. I use a wallboard joint compound, but there are lots of readymix filler compounds that work well. You want to find one that claims not to shrink when it dries.
The trick of applying this for our purposes is to place a... ummm... lump? Pile? Clunk? Anyway, put the stuff in the center of the road and use the putty knife to drag the filler across the roadway and up onto the grass mat. Work from the center out toward the grass, first one one side and then the other. If you feel like you pulled too much filler onto the grass, don't sweat it - you can cover this up in later stages with any of a variety of textures, including adding grass flocking back onto the board to narrow the road!
Then pull the knife gently along the direction of the road. If you like, you could drag a paint brush handle through the filler to simulate ruts in the roads. I want to be able to use my boards for different scales and the ruts would lend a definite sense of scale to the terrain so I opted not to do this.
Once the roads are done, you can break out a larger putty knife and work on the river banks. I use the same technique, basically putting down some filler along the bank and then blending it out toward the grass. There is no need to try and completely cover the foam since we're going to be adding yet more texture and paint. Just blend the filler as smooth as possible. You will always be able to hide any unsightly spots during the next step where we add rubble or you can sand out any rough spots if you prefer.. Avoid getting any of the filler on the river unless you want to model rapids!
Follow the directions on what ever filler you use as far as drying time is concerned, but remember that we've applied it thicker than it is designed to be used and it may take additional time to dry. The next step is adding additional texture. These things will really start to look cool after that!