Hey Franklin, good thing that wood is pretty forgiving. I suggest you just redo the notch you sanded down using an exacto or utility knife. You can use your photo as a guide for the position. Just don’t score too deeply into the wood, you can always adjust the notch even after the 5th string is in place. The square piece is part of the tailpiece. It should have 2 holes in the top, and you will screw the leather piece into it with the 5 holes for the strings on the top side. “Gourd side” means that side of the wood will face the gourd. Hope that helps.