As you work from the outside in, you converge on the center and eventually need to fill in the center of the window part.

Remember that some parts can scale both horizontally and vertically with the entire window, including lists, tables, multi-line edit fields, notebooks, and containers. Use these parts, if there are any on your window part, to fill in the center space, because they can scale to use all available space, regardless of how big the window part might be.

First, attach Label3 the same way you attached Label2: