If you can get a hold of the book, Dr. Bernstein’s Diabetes Solution, he described one way to determine how low one unit of bolus insulin will lower your blood sugar.
If you know how much 1 unit lowers you, you can figure out how many grams of carbs that covers using the table he gives that relates weight to how high one gram of carbs will raise you. For a 140 lb person 1 gram of carbs raises you 5 mg/dl (.27 mmol/L). For a 70 pound person it raises them 10 mg/dl (.55 mmol/L).
So if 1 unit of Novolog dropped you 45 mg/dl and you weighed 140 lbs, you’d know 1 unit would cover 9 grams of carb. If 70 lbs, 18 grams.
Then you start out matching the insulin to the carbs you are going to eat using a ratio that errs on the side of leaving you too high, rather than too low, ie. if you thought the ratio was 1 unit of fast insulin to 15 grams of carbs, you might start out with 1 unit and 20 grams, just in case you were wrong, so you’d go higher, not lower. Then you work your ratio down until you get to the right dose.
Use a food that you can weigh on a food scale so you know exactly how many grams of carbs are in it when starting out. It’s best to start with a modest amount of carbs, too-so you use no more than 2 or 3 units of insulin so if you are wrong you don’t end up with a big problem.
Once you have a good idea of the ratio, remember it isn’t always going to hold true, for example, some people will need more insulin at breakfast time because most humans are more insulin resistant on awakening. It will also change with illness and other stresses.
Obviously, if you are going to count carbs, you also have to learn how much carb is in your food. This means weighing out portion sizes as given in nutritional books or databases. The portion size is often FAR different from what you would think of as a portion. This step is EXTREMELY important.
A muffin is often listed as having a 2 oz portion size, but the ones sold at bakeries are often 8 oz. You can see where counting carbs could be a disaster unless you can learn to correctly estimate the weight of your food and keep aware of the portion sizes in the nutritional guides.
It seems overwhelming, but over time it becomes part of your brain and is no big deal. I’m usually very close without needing to weigh anything after years of counting carbs.