Here we go;
Good Old Dave
I use my basic lat pulldown high/low pulley for the ghetto ghr's. The guy in the picture is using toe pads at the bottom to hook his feet and a pad on the floor. My pulley doesn't have the toe pads so I put my knees on the actual seat and hook my heels under the knee pads. (I put an extra cushion on the seat since my knees are old and arthritic
). I put a plastic chair in front of me just cuz I have it around but of course any old thing at the right height will do. The other suggestion in the post is a roman chair.
The soreness you got in your quads on the step ups was probably just a function of the novelty of it rather than too much weight. If you managed that weight then don't lower it just add reps some reps, etc.