The reason I use felt is part of the process involves using a spray bottle to dampen the back of the mat with water. Water adds weight and allows for some stretching which will allow to mat to form tightly to the foam under-structure and create wonderfully smooth gradients. Too much water will cause the grass flock to fall off, but that's not awful as it leads to some variation in the tone of the mat. I'm going to be layering lots of additional texture so I can always tidy any spots that don't look good.
MKP was good enough to send me a pair of mats rolled in a tube at my request (the default is folded into a bundle).
It's basically just cutting out shapes a little bigger than what you need. I will trim them to fit after the glue dries overnight.
I use wood glue for this and pretty much just apply it to the foam and then use a damp brush to paint the entire surface of the land, paying special attention to the outside edges.
Below you can where I got part of the mat too wet (the lighter bits on the left), but as I said this isn't really a problem. The green of the felt still looks fine and I will be adding earth and static grass in places so when the boards are finished, you'll never notice.
I used straight pins to tack down any pieces where there was a difficult contour. They can be removed or simply covered over with later texture and paint. After leaving the boards to dry overnight I used a knife to neatly trim away the parts I didn't want and clean up the edges.
Using a marker I define where my road and any other sections of exposed earth will be. There are detailed instructions about this process here.
The next step is to use joint filler to seal the rest of the exposed foam and blend the river banks and roads into the grass and I'll have another post later this week...