Here’s a SUPER-cheesy-easy way to get started without the hassle of writing an editor:
make a little piece of geometry (like a tiny colored sphere or cube) and place instances of it into the scene to indicate walkability.
For each walkable cell:
—> Duplicate your marker sphere to place into each walkable cell
—> Drag it laterally with the collide-against-ground modifiers (CMD-SHIFT on Mac) to track the ground
—> optionally snap-place it (I wouldn’t bother initially, you can write a script later)
Once you have the walkability defined for a small test scene (like 10x10 cells!), read in all those markers (either name them specially, put a tag on them, put a marker script on them, child them all under a special GameObject, whatever you like) and use their integer cell positions to mark off your cell as walkable.
After that you can start writing editors to “paint” big areas, or rectangles, or whatever.
It’s unfortunately a bit daunting to get started, but once you get started you can just keep iterating on improving your editor making it do more stuff. When you make a real editor, just import all the above work into it.
I highly recommend using source control and saving and committing your work REGULARLY!!!
Personally I use git because it is free and there are tons of tutorials out there to help you set it up.
As far as configuring Unity to play nice with git, keep this in mind:
Here’s how I use git in one of my games, Jetpack Kurt:
Using fine-grained source control as you work to refine your engineering:
Share/Sharing source code between projects:
Setting up the right .gitignore file:
Generally setting Unity up (includes above .gitignore concepts):