Click on the ceiling to which you wish to add the false ceiling. In the properties panel that opens in the left, select the Create False Ceiling. You will enter a mid-state where you will be presented with the ceiling plan view. Click on the Add option in the upper ribbon to enter the Sketcher mode.

Once there, draw the desired shape of the ceiling, and click on Apply to save changes, or Discard to ignore those changes. Then click on the Back to Designing arrow at the top right corner to finalize the ceiling.

Click on the false ceiling to wish to edit. In the properties panel that opens in the left, select the Create False Ceiling. You will enter a mid-state where you will be presented with the ceiling plan view and the false ceiling shape in grey.

Click on the shape to select it, and click on the Edit Shape Option in the properties panel on the left. You will further enter into the Sketcher Mode where you can modify or create a new shape. Ensure you create CLOSED polylines/curves only. Once done, click on Apply to save changes.

In the mid-state, click on the ceiling shape to select it. In the left panel, under the Material menu, click on the Select Material drop down menu and click on one of the user-defined materials therein, or click on the Fetch From Store icon at the right of the Material menu to open the carousel and select a material from there.