The basic thing to keep in mind is that you can't cope with sugars and starches, not to the extent considered normal these days.
You can eat protein and fats, so meat or fish, shellfish, eggs, cheese, cream, butter, lard, coconut oil, olive oil are all fine.
The seed oils PUFAs, or polyunsaturated fatty acids are not good.
Avoid sugars - even ones which claim to be healthy - honey, various syrups, fruit juices, are all just the same to your body, high sugar fruits are going to cause spikes - but many people find that a small serving of berries with yogurt or cream are OK - I eat that as a dessert a couple of times a week.
Starches are anything made from grains, breakfast cereals, bread pasta etc. Potatoes are high starch as are many root vegetables, rice and oats are usually off the menu too.
Salad stuff and above ground vegetables, mushrooms, and anything with 10 percent carbs or less are usually safe. I add in high cocoa chocolate in small amounts.
It seems to be a diet many people thrive on and the lucky type twos can find that they get normal numbers and even lose weight as a bonus.