A lean bulk is a muscle-building phase in which you try to build fat-free muscle. You achieve this with a low to moderate calorie surplus of about 100-300 kcal.
In the long run, you'll build muscle with much less body fat than in a bulk with a very high calorie surplus. Calculate your calories, train hard, and keep the calculated moderate calorie surplus constant. If after two weeks you have not gained weight on a weekly average, increase your calories by 150 kcal by eating about 37.5 g more carbohydrates.
If you are a relatively thin person with low body fat percentage and have not gained weight after two weeks, you can increase your calories by 250 kcal by eating about 62.5 g more carbohydrates.
However, if your weight gain is more than 0.5-1% in a month, you should adjust your calories downward. In this case, reduce your calorie intake by 150 kcal. In addition, make sure your protein intake is 2 g per kg of body weight.
A lean bulk has a significant advantage in that you do not put on too much fat too quickly. Thus you can make your muscle-building phase longer without considering a diet. In the best case, you put on as little fat as possible during your bulk. However, it is almost impossible to put on no fat at all.
If, after a certain period, you have put on too much fat for your liking, you can do a cut. Often a mini cut, i.e., a challenging but short cut over a few weeks, is sufficient because you put on less fat in the lean bulk than in the classic bulk.
Furthermore, you should train hard and achieve progression in your training. Progressive training with a slight calorie surplus and an optimal macronutrient distribution ensures effective muscle building.