I don't know if this has already been mentioned, but I just learned about it today & thought other people might want to know!
I kept seeing screenshots of islands on tumblr and pinterest where people had placed custom designs and gotten rounded corners as you can do when placing the default paths, i.e., cobblestone, sand, etc. I kept trying to place custom designs using the path tools and it wouldn't work the way I wanted it.
So, turns out you need to place the default paths, like cobblestone or dirt, in the shape or design you want. Then if the custom design path you actually want has a transparent pixel in it (you leave one square transparent in the corner or somewhere that wont be noticeable) and you place this down over the default path, it copies the placement of the path under it!
Here's an example of what I mean. See how it's a custom design but has rounded corners like you can do with default paths?

I can't be the only one who had NO idea this was a thing so hope it helps someone else too!
I kept seeing screenshots of islands on tumblr and pinterest where people had placed custom designs and gotten rounded corners as you can do when placing the default paths, i.e., cobblestone, sand, etc. I kept trying to place custom designs using the path tools and it wouldn't work the way I wanted it.
So, turns out you need to place the default paths, like cobblestone or dirt, in the shape or design you want. Then if the custom design path you actually want has a transparent pixel in it (you leave one square transparent in the corner or somewhere that wont be noticeable) and you place this down over the default path, it copies the placement of the path under it!
Here's an example of what I mean. See how it's a custom design but has rounded corners like you can do with default paths?

I can't be the only one who had NO idea this was a thing so hope it helps someone else too!