Draw in the canvas area to the left. You can also use the browser's inspector to tweak the SVG.
This is not a full featured object editor. There are already a lot of them, plus it is pretty easy to manually write SVG for basic objects. This tool is probably best to generate a few "natural" paths, then copy that into a larger svg document or as a small glyph.
Don't forget you'll be able to resize it with the display, so you can draw it as large as you want.
There's a known bug with getIntersectionList, so the erase tool requires you to click on each line you want to erase.
The color input isn't working both ways.