I deleted your link. This is a kid-friendly Family show.
In the illustration below, I used the ‘Edit Node’ tool to insert a node (hover over a location, press “I” (capital i) at the intersection points of the top line. Then hovered over the small segment on left and hit the “D” key to delete that segment. Notice the cursor is now hovering over the small line segment on the right, ready to delete next.
If you then zoom in to each node location, grab and move slightly and without letting it go, move it back and then let go. The node will ‘snap’ to the one under and connect into a single resulting node. Continuing through the remaining nodes will result in a closed shape.
You can also use the ‘Boolean’ tools to snip off the corners as well. Have a look at this to get an overview of when and how to use each.