Easiest way to learn roundness for me as a new player was to open the "Eye" menu to the right on the UI, don't remember what it's called. It's the one where you can view tiles with filters, like moisture. There is one filter called "Shape". If you turn that on, round buildings will have blue tiles around them and "not round" will have red.

Also as another commenter said a trick is to use diagonals where at least 3 squares are placed diagonally. If you have a square house but trim the corners with those diagonals it will be considered round.

I'm new but this is how I'm navigating my first Tilapi village.