I had mine done today. In short, the rear bumper has to come off and there are templates for drilling the holes as well from the inside out plus extensive instructions from Toyota. It took a couple of hours and the sensors work very well.
I know the dealer's tech well and trust him implicitly and the fit & finish is perfect but he said the template isn't the best so easy to get it wrong. The holes for the dealer fit kit are not pre-drilled and the sensors are different than the factory fitted ones on the USA cars but the bumper has the markings on the inside for the factory kit but I actually think they look more pronounced (see the last pic for comparison).
I had originally bought the official kit from the dealer and they painted them before other forum members mentioned about Toyota's good will of paying for them. I sent TOL an email last week and they agreed to refund me the cost without any hesitation so fair play.
Pic below of the part numbers of the kit that might be helpful for others
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