The sinplest way is to build up the different levels with basswood...stack it up higher where you need it high, like the sidewalk...and it comes in straight strips so you can get a nice straight kerb without much effort. And seal it with something waterproof like auto primer before you put any gloopy stuff on it.
If you want to get into the finer details, then you will have to consider the crown in the centre of the road and the slope of the sidwalk towards the gutter. When you really look at it, nothing is flat, so keep that in mind and your groundwork will look a lot better.