This pic shos at the bottom the fence, in the middle the cutter with the blade at its highest point with respect to the fence and beside that a piece of the ply that will join to the slotted piece. At the top is a metal straight edge.
needless to say this is done with the router OFF !! To achieve this the fence is adjusted accordingly and then clamped for ALL cuts of this size.
If you look closely there is a tiny gap between the ply and the straight edge which means that the inner edge of the slot as we cut it is just a hair more than the ply thickness. This translates into a tiny excess of the exposed edge which we will later on either sand to flush or use the router with the guided cutter to do the same job