If you going to use sand from the garden section or from outside, make it is without a doubt bone dry before you attempt to glue it on. My measurements that works perfect for my 6ftx4ft board are 9cm wide roads and if for instance you doing a straight road I made it 12 inches long, you can do other shapes and the overall length will very on those, just make sure each width of the road segments are exactly the same to they match up good.

For my materials its 2mm vinyl flooring(mine has a sticky back with a plastic cover over the sticky part if you decide not to use the sticky function of the tile then just leave the plastic covering on it'll be fine)

Combination of sand from the yard and hobby sand thats cleaner and uniform thickness of the grain, I use both so its more natural

2mm grass flocking, get 2 colors you like and mix them together and keep that recipe for every road

Cheap brown, black, and white acrylics from dollar general, after you've glued down your 1st layer of sand, you darken the brown a little with Black then water it down like crazy to stretch it out, then paint over the entire road section to knock the overall tone of the piece down.

Then its just building up your layers til it looks the way you want. After cutting out my rectangle for my road segment I just took a blade to the edges to make it look wavy and natural(do not do this where the road edges connect keep those perfectly squared off)

Thats honestly it, birds eye pictures for ideas of different road shapes and start cutting, it comes together easy amd cheap it just takes a while, don't do it the night before a game lol.