Cut thru problems(new to blender)

Hi,
just started doing different toturials, and i am having problems with cut thru option.
i cannot Seem to get the cut to go directly thru my model. I tryed on the starter box too.
it seems it follows a weird path when i try to get it from Corner to mid point.
the top cut is fine but on the other side it goes in random directions. What am i doing wrong?

If you are cutting with the knife(K) tools then it works like this :
hit K -> Z hold down -> cut (drag the mouse) -> left click to confirm and enter to cut

You are ending the cut in the middle of a face, knife tool will try and connect the cut to the closest edge.
Either end the cut in an open space or on an edge.

Use an ortho view. Add a plane to your object, scaled up larger than the object. Subdivide opposite edges of the plane. Then cut through, from one edge midpoint to the other.

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