I want to cut out the shapes seen in the background of the image below, onto the plane:
I want to do it symmetrically, preferably using the knife tool. The mirror modifier won't work here; I only want symmetry on a specific face, or specific faces, of the object that I'm currently modifying in edit mode. I have already tried using "X mirror" in edit mode options. How can I accomplish a result like this?:
Note: I already am using mirror: