Create an Editor Tool That Uses a Brush

I need to add brush functionality to an editor tool. It is a simple tool that will take a brush and draw various colours to a file.

I have no idea where to start on adding brush functionality. I need a brush simular to the one used in this video.

I have seen plenty of tools around with brushes, but can't seem to find any free ones in the asset store.

If someone could link me a tool that I can pick apart, or some documentation to get me going I would appreciate it.

Thanks!