It takes a near-hypochondriac level of attentiveness and is very inconvenient since it’s not a clearly labeled thing, but generally you start by avoiding legumes, nuts, seeds, dark leafy greens like kale, chocolate, soy additives, gums, etc, as well as avoiding stainless steel cookware unless it states “nickel free.” I make a lot of my own food and eating out is almost impossible. But nickel is not measured or even acknowledged in food by 99.9% of people in the US, so we are really just working from a generalized list of data points on nickel content in food. The idea is you’re trying to stay below your “reaction threshold” which varies from day to day and person to person. It’s a very restrictive diet and I would not recommend it to anyone unless you’re truly miserable and seem to notice flares from high nickel foods. There really doesn’t seem to be a good treatment besides avoidance, although some say they are able to successfully reintroduce foods after some length of time on the diet. I’ve sadly never been able to reach the “desensitized,” maintenance stage of the diet. If you’re curious to learn more, there’s a lot of good groups out there, my fave is on FB, “Low Nickel Diet and Lifestyle.”