How to start

Keto is a low carb and high fat way of eating. My favourite meal, for example, would consist in yummy ribeye steak, rare, fried in butter and served with sautee spinach with blue cheese. It takes 10 minutes to make.
"Give a man a fish and you will feed him for a day, teach a man how to fish and you will feed him for a lifetime."
This is me, hopefully, teaching you how to fish.

I would like everyone to see this a lifelong change. It's your new way of eating.

I am not going to give you a food plan, though I will support you with a list of foods that are naturally keto friendly. There are plenty of these list on the internet.
I will try and pass my knowledge of keto and where you can go to confirm that what I am saying is not made up. I will give you tips, hints and support. But you need to want to make any changes and do some of the thinking yourself. Consult a doctor if you have any concerns. It's definitely worth having him or her on your side.

This is what I did and every time I am not feeling my best I go back and retrace some of these steps.

1) Start by cutting out any carbohydrates now (sugar, grains and starchy vegetables).
You want your carbohydrates to be mainly incidentals, meaning as low as possible. Avoid anything that will bring carbs to your plate without any meaningful nutrient profile.
Out of the three macronutrients, carbs, protein and fat (actually alcohol is a 4th macronutrient but irrelevant), carbohydrates are the only ones we can live without so whatever little you will get from your vegetables is more than enough. The body can make more glucose out of amino acids and glycerol for example via a process call gluconeogenesis as required.

2) Calculate your macros [optional].
It is not necessary. I mean, I can't imagine cavemen worrying too much about the percentage of carbs, protein and fat they consume on a daily basis. And many people that follow keto don't think about macros. With that being said you should re-educate yourself on what you actually need to achieve ketosis.

I used this calculator I found on Reddit but there are other websites like KetoDiet, that offer their own. They all do the same.

Most of these calculations are based on Volek and Phinney's research.

They worked out that carbs should be limited to below 30g of carbs. Most will keep it under 20g. This will force your body to switch fuel from glucose to fat.

Protein should be in proportion to your lean body mass to ensure you maintain muscle mass but not too much that would impact ketosis, since protein will trigger some insulin secretion. The protein required is usually calculated as 1.3g to 2.2g per kilogram of lean body mass, depending on your activity levels. In my case my lean body mass was 70kg (102kg with roughly 30% body fat). If you multiply 1.3 you get 91g of protein minimum.
The remaining of the energy/calories should come from fat. Either from your plate or from your body.
Calories are not really that important. As a percentage of your total calories intake, if you monitor calories, your macros will come to: Carbs - 5%, Protein - 25%, Fat - 70% fat.
Carbs = Limit, Protein = Goal, Fat = Satiety.

3) Track your food. [optional].
Another tool that is not strictly needed but helps. It is actually linked to step 2.
There's sugar EVERYWHERE, you may be having too much carbs unknowingly.
By tracking I mainly mean carbs and protein. Your main job is to ensure your body learns that sugar, starches and grains are not an option.
So, if you're hungry eat fat. If you track your food you will be able to plan and choose wisely.
Chances are, you have had an unhealthy relationship with food. So in the beginning it helped me staying accountable by knowing how much actually carbs and protein were in each food item. Knowing this has helped me reach ketosis.
Your macros should be able to help you stay accountable.

4) Eat when hungry, stop when full.
Our bodies were designed to eat when you need to and then go without for long periods when we don't.
You will learn that fat is very satiating and eventually you will not be hungry as much. This is one of the best things about keto. The freedom from food.
In fact, though calories shouldn't matter, it helped me not going too low since it could reduce my metabolism and have the opposite effect of losing weight. It also helped me understand what I was eating in comparison to how I was feeling and hunger spikes. Meaning if most of the calories were not from fat, I would usually feel hungrier sooner.
Once I was fat adapted, and ate only when hungry, I noticed that I could go the whole day not feeling hungry. That's when I started intermittent fasting but that's a topic for later.
Just know that calories don't matter much. You can go over, if you do let it be from fat. Eventually it will all level out, even f you gain some weight initially. Just listen to your body.

5) Increase your salt content.
I know, you have been told to avoid salt. But that is because on the standard diet your body will naturally retain water.
Salt is needed. That's the end of it. On a low carb diet your kidneys won't retain as much water and you may lose electrolytes as a result, sodium being one of the most important. The other ones are potassium and magnesium, both you can easily get from green foods.
You could take supplements but I didn't. Just like macros, cavemen didn't take supplements either.
Just increase salt and if you feel dizzy or have headaches, increase some more.
Sugar is often compared to a drug. And your body will go through withdrawls. We call it the keto flu. Salt will help you conquer it.

6) Keep it simple.
Your sole job in the beginning is to get rid of your body's reliance on carbohydrates.
So, keep it simple. Eat green leafy vegetables as they are rich in potassium, iron and magnesium. Eat fat like butter, coconut oil, olive oil and animal fat. And excellent quality meat. Any meat. If you eat fatty meat, you can use the fat drippings to cook your vegetables in for extra flavour.
Keeping it simple will make it easier to control and track what you eat.
Once you're used to portions you can even forget about tracking and eventually you won't need to control it either as you will instinctively know.
You will go through a period of adaptation where you will get to know real signs of hunger and satiety

7) Keep yourself informed. Research and ask questions.
Keto is still controversial for many doctors and the idea of fat goes against what we have been told for the last 40 years.
Reading about science, following doctors that advocate low carb and share studies, discussing with others and listening to podcasts helped me keep motivated and informed.
Here's some of my favourite source of information and I cannot stress enough that you should go have a look:

When I was doing it alone reading about it was enough to keep me motivated. But some of our friends struggled as they were not as inclined in understanding the geeky stuff.
So, my wife and I started a small Facebook group where I would share some of the science and answer questions while she would concentrate on experimenting with food.