Mesh distortion is a nice and easy feature that let's you interact with your frame's sides with the help of some special handles.
To modify a frame or a shape, just select it and go to Item | Path Tools | Mesh Distortion.
A window will appear. The selected frame or shape is displayed inside a grid too and has some circle handles.
Dragging those handles will distort the grid and the shape as well. Take the third handle of the second row and drag it down.
Take the third handle of the third row and drag it down too.
You then just have to adapt other handles to make them nicer and smoother.
Dragging handles just distorts the shape by the same amount as the grid. The current active handle is displayed with a red fill, whereas the inactive handles are kept with a single red stroke.
By watching the position of the handles or the shape of the grid, one can see what changes have been made. In the example, we have mostly moved the two middle handles...