The good news is there are things you can do to help alleviate those symptoms. Firstly, you should see your doctor before going on the diet to get the thumbs up. Then, if you're experiencing such symptoms and they're not the result of something else (like the actual flu), start by drinking plenty of fluid, while avoiding activities that can dehydrate you such as an intense workout. Dr Evelyn Lewin also suggest increasing your salt intake by adding half a teaspoon to two teaspoons of salt into your food. if you're able to persist through these symptoms, you’ll quickly transition through this stage and come out the other side bursting with energy.
While macros will differ a little from person to person, the general rule of thumb for keto is to keep carbohydrates under 5% of your daily caloric intake. As long as you avoid the foods mentioned above, you should be fine. Google “TDEE calculator” if you need some additional guidance on how many calories to eat. I’ve had success following this way of eating as it allows me to eat foods that taste great. There are tons of resources online as well if you need additional guidance. A quick google search should turn up a ton of resources. Hope this helps!