How To Gain Weight?

Please i need tips on how to gain weight.

most people are complaining about how slim i am. the truth is i rarely used 2 eat but i've started eating now but i  have a problem with dinner, anytime i eat esp after 7pm, i usually have constipation, which is not workable since i work.

How do i gain weight without losing my flat tummy please?

how slim is your slim?
what is your body mass index?(BMI)
some tips on gaining weight the right way are :

Calories: Our bodies have a need for a set number of calories each day. This need is based on your age, gender, height, weight, and activity level. In order to gain weight, you will need to take in more calories than your body needs to maintain your current weight.

Nutrients: There are six essential nutrients that our bodies need to function; they are protein, carbohydrates, fat, vitamins, minerals, and water. Each of these nutrients has its own function and is needed in different amounts. The way to get them is through the foods that we eat, and in some cases, by the supplements that we take. Your goal for gaining weight will be to eat a well-balanced diet in order to get all of these nutrients in. The food groups that you will want to focus on increasing are:
Starch – This is a great group to get calories from. Try to eat foods made with whole wheat flour. Have bread, rice, pasta, potatoes, cereals, beans, and starchy vegetables (corn and peas) with all of your meals.
Fat – You will get a lot of calories for a small amount of food when you add fat to your meals and snacks. The goal here is to avoid the foods with the trans fat and have the ones with little or no saturated fat. You can use nuts as a snack or add them to your meals, cook with olive or canola oil, and use peanut butter as a snack or with your meal. It's important to get foods with omega-3 fats, so include fatty fish (salmon, albacore tuna, mackerel, lake trout, herring, sardines), flax seeds, and walnuts. Be careful not to go overboard with fat because too much is not good for your health. Stick with the guidelines of less than 30% of your calories coming from fat.
Meat - The protein in meat is great for building muscle. Foods in this group includes chicken, turkey, seafood, eggs, pork, cheese, and beef.
Diary – Be sure to include low fat (1% or 2%) milk and/or yogurt into your diet. You can add some nuts or flax seeds to your yogurt to have as a snack, and you can have a cup of milk with your meals for the added calories.

Fruit – There are lots of health benefits to fruit, and you can have them with some of the other foods to boost the calories. Fruit with peanut butter or cheese are great snacks with lots of calories.

Vegetables – As always, keep your vegetable intake up for the great health benefits that they provide. Your diet needs the balance of each of the groups for optimal health.

get a frequency:
It may be helpful to eat every couple of hours in order to get enough food and calories in. Set a timer on your cell phone or put it in your appointment book to remind you to eat.

supplements:
If you can't get the calories in from food alone, you can consider using a supplement.

Ensure in between meals

Adequate rest and enough sleep.