For Compare by Drawing, two substructures have to be defined. One of these gets the option "second structure" by right-clicking on any atom of the substructure and selecting the checkbox "2nd Structure" in the Atom Attributes window. The second structure will be shown in red.
Structures can either be drawn directly in the editor-window or can be inserted into the editor-window by drag and drop or can be imported as a MOL file
The below graphic shows two substructures to be compared, one the second of the two 'C'-atoms have been replaced by 'O'-atoms.
From 45 hits the list of compound pairs gave, one is shown below.