Ok, there are a lot of things I'm doing in the video which aren't entirely clear, since I didn't narrate it, and I haven't figured out how to make captions yet. So I'll try to explain. Let me know if I need to explain something else.
When you drag the mouse from a ruler on to the canvas, you create Guides. Guides make excellent snap targets, which I use often to help with alignment. Also notice how I have the snap control bar set up (right side of the window). I've made my guides blue, but I think they might be red by default.
When I'm zoomed in, and you see a new node suddenly appear, I do that by double-clicking with the Node tool. Position the mouse over the path, and double-click, to add a new node.
Notice when a node is selected, there's a thin line with a tiny circle on the end of it. That's a node handle. When you see me put the mouse in that tiny circle, and the circle disappears, I've deleted that handle. That makes the path straight. To delete a node handle, the node has to selected, you have to put the mouse in the tiny circle, hold the Ctrl key, and click.
Here's the video:
Let me know if I need to explain something else. I'm not sure how much you already know how to do.
