Before you start guzzling down weight gainers and three servings of protein powder a day, we would recommend evaluating your current diet first. The easiest way to go about it is to use a food logging app such as MyFitnessPal to analyse the macronutrient composition of your diet. This will save you from having to log food and calculate macros manually.
These apps have plenty of food items already added to their huge library; all you have to do is scan the barcodes with your phone's camera and tell the app how much you ate. The app will then calculate the macronutrient (carb, fat and protein) percentages, as well as other useful stats, like how much sugar, salt and fibre you have consumed.
Once you get into the habit of logging, you can set goals for your daily intake: let's say you want to cover your daily energy needs with 65% carbs, 25% protein and 20% fats. The app will tell you how many grams of carbs, protein and fat this translates to.
This is where weight gainers come into play: when you see you're under your protein and carb goal, you can consider adding a serving of weight gainers to your daily intake to help you boost your daily macro numbers.
It might sound a bit confusing, but it really isn't all that difficult in real life. You need to be a bit more mindful of your diet, but you should be anyway if you aim to gain lean muscle mass effectively.
For the best results, use the recommended serving size if you want clean gains. We highly recommend mixing weight gainers with good old tap water. You could potentially use milk/milk substitute should you want to, but 500 ml of milk will add an extra 200 calories and add 25 grams of sugar to your diet. It is just unnecessary.