When you add your screens to the flow, they are brought in based on their positions in the design file by default. Then, you can visually lay out your screens within the flow.
To move them around, you can simply click and drag. Unlike a design tool, screens snap to a grid to align them easily. You can also select multiple screens and move them by dragging them around.
Here’s an example where the screens are laid out based on two different paths that a user can take: