Dehydration and carb withdrawal.
As you are first starting, you can and may want to drink a lot of water each day. When you feel hungry, drink water, when you have a headache, drink water. Over time, your body will balance out and you won’t need the water.
And as a side note, it may not be a great idea to go cold turkey into it. You can always spend a week lessening your intake each day until you get towards your goal. There is no shame in that, and it can be much more healthy.
Lastly, a pre-prescribed amount of nutrients (x amount of carbs and x amount of this and that) may not be healthy either. Every body is in a different state of health when they start and you may have to adjust it for your self to get an idea of where you want to be.
Ketosis is simply the process that happens when you haven’t eaten in awhile. You can easily start by eating an early dinner (4-6pm), sleeping and then trying to not eat for as long as you can. You’ll be in a state of ketosis if you last past 10-11am. If you make it til around 1pm, you’re doing awesome. — do that for some days and learn the ropes, slowly pushing yourself further and further and learning what is useful to eat or not to promote your lasting longer. Eventually you will feel comfortable enough to experiment more with it.
To just blindly follow detailed instructions from the internet could be slightly harmful or at least scary.