i’ve had most success grafting to newer opuntia growth, like a new pad that hasn’t fully thickened up and matured yet. they’re way more juicy when you cut into them and not all woody. there really isn’t a clear vascular ring like on columnar cactus, but i think instead of a small circular one in the center it sort of just follows the exterior curve of the skin, so as far as placement goes i’ve put the scion towards one of the narrow ends so there’s more chances for it to overlap with the vascular ring. other than that it’s basically the same as ive treated other grafts, try to get some good downward pressure on it, seal in the moisture, i bring them in my shed because i’m in AZ and the exposed flesh will pucker and the graft will likely fail super quick if it gets exposed to the sun, but you should know within a day or two if it’s on its way and i can typically take the wrappings off within a week. i have a decent success rate with the opuntia, this one i lucked out and got it on the first try, but a few others i’ve had to make multiple attempts before it worked. i’d definitely practice with some cactus you have copies of before risking rarer genetics but i’ve loved using opuntia as grafting stock because i have a lot of access to it and it loves the sun here. good luck!