I took the bumper off. I bolted my recovery points on the inside of the C-channel frame rail...so I know the area was right next to the mounting flange of the bumper...minus the thickness of the frame rail (1/8" or so). I had enough dirt that left a clear marking of where the frame rail was, so I did very little marking.
Angle grinder with 4 1/2" cut off wheel. I cut the longer vertical slits and then an X in the middle kinda...the width is very narrow (3/4" or so plus some slop). It is difficult with a 4.5" cutter disc...but can be done if you notch it and angle it enough. Or use a dremel on the last narrow cuts.