Elements Context Menu
At any time when not focused on an input field, you can press either 'e' or 'f' to bring up the Elements Context Menu. This will show a list of all elements under the cursor and allow you to create a component from it. First, find the element you want to create a component for, then click once. You will see a list of selector combinations for that element. Choose the selector you want, and a component will be created. If you pressed 'f', the component will be auto filled with all elements that are children of that element.
You can also create components directly from the Component Library (press 'c' to toggle visibility)
Next, let's look at the Component Builder itself.