How do I correct my blood sugar level with an insulin dose for diabetes?

Extra insulin taken because of a high blood sugar level before a meal is called correction insulin. Just as you need to establish an insulin-to-carbohydrate ratio, you should try to figure out your individual correction factor, the approximate fall in blood glucose level that you expect from a unit of insulin. Obviously, no one can ever know that number exactly because it will vary somewhat from one situation to another. But with a little trial and error, you can usually figure out that 1 unit of insulin will lower your blood sugar by about 25 points, by 30 points, or by whatever you determine to be your number. You'll have to go through some trial-and-error testing, keeping careful records, just as you did with the insulin-to-carbohydrate ratio for food.
For most people, it's reasonable to start with the assumption that 1 unit of insulin will lower the glucose by 50 points and then set a target for the upper-limit premeal number you'll accept. For example, you might decide to correct for any glucose number over 120 before meals and assume at the beginning that 1 unit of insulin will drop you by 50 points. If your premeal blood sugar level is between 121 and 170, or up to 50 points above the 120 mark, you'd take 1 extra unit of insulin; if it is between 171 and 220, or between 51 and 100 points above the 120 mark, you'd take 2 extra units; if it is between 221 and 270, or between 101 and 150 points above the 120 mark, you'd take 3 extra units, and so on.
If your correction insulin isn't lowering your sugar as much as it should or is lowering it too much, you'll have to adjust your correction factor up or down. You'll have to try 1 unit for every 30 points of glucose above your target or 1 unit for 25 points until you can reliably reach the proper range. You may have to subtract some insulin from the amount you would normally take if your premeal number is low.