When you cut carbs your body sheds water and, with it, sodium, potassium and magnesium. Replacing them is what actually fixes the keto flu. Here are sensible daily targets. See also electrolytes on keto.
Sodium is the big one and the fastest fix for keto flu. Salt your food generously, and a cup of broth is an easy way to top up.
General daily targets for a healthy adult on a low-carb diet, not medical advice. Keto raises sodium needs well above the usual "eat less salt" advice, because low insulin makes your kidneys excrete more of it. If you have high blood pressure, kidney disease, or take medication that affects electrolytes (including diuretics), talk to your doctor before loading up on salt or potassium. Potassium supplements in particular can be risky in large doses, so get it from food where you can.