Click on the existing tiling to which you want to add another tiling, and go to the Tiling and Cutting Tool option in the lefthand panel.

You will then enter the mid-state, where you can see only the shape of that tiling created colored in grey, and the outline of its base wall.

Once there, click on the Add option in the upper ribbon, to add another shape. You will enter the Sketcher mode, wherein you may then sketch the desired shape. Click on Apply to save changes and return to the mid-state.

The tiling most recently added is the one that is shown over the others in the mid-state. To move the tiling above/below the other, click on the tiling shape and go to the Move Up/Down drop-down menu in the left panel. Then, click on either the Move Up or Move Down depending on whether you want to move the tiling above or below another.

Bear in mind that the Move Up/Down tool only affects the visibility of the tiling's two-dimensional shape against the others for ease of selection and editing, and does not affect its actual position in real-time.

Click on the tiling shape you want to delete, and click on the Tiling and Cutting tool. In the mid-state, select the targeted shape, go to the Delete Shape option in the left hand panel and click on it to delete the shape.

Once you are done editing the tiling, you may click on the Back to Designing arrow at the top-right corner, to return to the workspace.