That’s a really hard set of questions, because the answers keep turning into long stories that I have to erase because of TL;DR. It’s still too long, but this is what I have for you.
I was misdiagnosed type 2 but didn’t believe it, got an endo appointment a month later, the endo ordered antibody tests and the day they arrived she turned me over to a type 1 CDE for insulin instruction. The CDE talked to me for about an hour, gave me starting numbers for basal (lantus) and insulin:carb and correction ratio for bolus, and sent me home with prescriptions for Lantus, Novolog, and syringes. I got 8 test strips and 4 syringes per day. I started basal immediately, and on my own adjusted it for a week until I stayed more-or-less level. A few months later I split it to half in the AM and half in the PM just on the theory that it would provide a smoother dose. I kept a log book to record all insulin doses and eating so that I could try to observe cause and effect.
Then it was Thanksgiving day, so I started bolus insulin. I guessed a number of grams of carbs, took the shot, and waited for the meal to arrive.
First lesson learned: I have to know when the food will actually arrive before I take the bolus, because if it’s too late I’ll go low.
Second lesson learned: I have to actually see the food before I guess how many grams of carb will be there, because if it’s a restaurant meal and it turns out to be smaller than I guessed, I’ve just overdosed insulin. Of course at home I weigh the food and multiply by the %carbs of that kind of food, either from the nutrition label in prepared food, or from the USDA nutrient database (a PC program initially, now “Food Nutrients Finder” on iOS.)
Third lesson learned: if I overdose insulin, it’s really easy to fix, just eat a good number of glucose tablets. (a) I have to have glucose tablets with me. (b) I estimate 10mg/dL rise for each glucose tablet, and eat the number that will move me from my current BG to 100; if I’m falling fast I’ll eat a couple extra for safety, if I’m severely low I’ll eat 5 extra to rescue myself before I pass out.
I say third lesson learned because I hadn’t been properly taught that. When I realized I had overdosed for dinner I ate some bread, but it wasn’t enough. When I started shaking violently I was taken for medical help. It appears that I never actually went below 120 mg/dL, but the rapid drop had a dramatic effect on my insulin-naive body.
I tested myself to find the correction ratio (how many mg/dL do I drop after 2 hours when I’m fasting, steady, and I take 1u insulin.)
I adjusted my insulin:carb ratio until I was about as likely to end up too high as I was to end up too low. This was 8 years before CGM, so with a limited number of fingersticks, there was no way to sugar surf.
After a while I realized that it wasn’t just a matter of taking the right amount of insulin, it’s about matching the speed of action of insulin to the speed of digestion, depending on the kind of food. To bolus for especially slow carbs I used a split bolus (partial dose of insulin to start, additional insulin after an hour or two.) To bolus for especially fast carbs, I bolused earlier ahead of eating and took an overbolus, with extra carbs later on to compensate for the overbolus. All this was just trial and error, gaining experience until I mostly did it right. That was a couple years.
Before CGM I also learned a lesson about basal insulin. If I wake up in the middle of the night, and my BG is too high, and I’m sweaty wet, it doesn’t mean I needed more basal. It means I had too much basal, went low in my sleep, my body had a panic and dumped hormones to save me, and that made a BG spike.