There is two way I have used insulin,
Basal insulin - long-acting insulin added to a calorie surplus, assist when your insulin sensitivity reduces during a bulking phase. Lantus is the go here. AM blood sugars is a must.
Pre-work out 30mins before - use to shift more nutrients into your muscles during training. Intra workout carbs here is a must, need to work out your ratio of insulin to carbs. Safe is 20 grams of carbs for every 1iu Novorapid, conservative would be 15 grams of carbs per 1iu, risky is 10g carbs per 1iu. Tritrate up over a week, pre and post-training BSLs are a must. BSL 4-8mmol is good.
Pre work out meal, don't train fasted, have your Intra carbs premixed and sip throughout your work out, and post-workout meal post training 30-45mins max. Insulin is active for about 3 hrs.
Have 50-100g of lollies in your training bag, and have a glucometer.
Get this wrong it can get really bad!