This article describes the steps using Web Editor. If you are using the RoomSketcher App, see Add Furniture or Wall Items on Top of Each Other (App).
It's easy to stack furniture on top of each other (for example placing a lamp on a table) and also add wall items like paintings and wall-hung televisions on top of each other.
In Properties, adjust the Height Above Floor to get the correct height.
To learn how, watch this short video: