If by “get fat” you mean gain weight, the most effective and safe way is to eat more calories than you burn every day, using mostly nutritious, higher‑calorie foods and spreading them across more frequent meals.134

Here’s how to do that in a healthy, controlled way:

1. Create a calorie surplus

• Aim for about 300–500 extra calories per day for slow, steady gain.149
• If you want faster gain, up to ~700–1,000 extra calories per day is often suggested, but this is more likely to add fat quickly.1
• Over time, a surplus of ~500 calories per day typically adds about 0.5–1 lb (0.25–0.5 kg) per week.279

2. Eat more often

• Eat 5–6 smaller meals per day instead of just 2–3 big ones.34711
• Schedule snacks between meals and possibly one before bed.346

3. Focus on calorie‑dense foods

Include foods that have a lot of calories in a small volume, so you can eat more without feeling stuffed:

• Healthy fats (very calorie‑dense):
  • Nuts, nut butters, seeds, avocado, olives, olive oil, canola oil.4101112
  • Add them to salads, toast, smoothies, or on top of meals.341011
• Full‑fat dairy:
  • Whole milk, full‑fat yogurt, cheese, cream, ice cream (in moderation).341112
  • Use milk instead of water for cereal and oatmeal.10
• Protein sources:
  • Eggs, fish, chicken, meat, beans, lentils, hummus, tofu.41012
  • Add cheese, sauces, or oils to these to increase calories.2310
• Carbohydrates:
  • Rice, pasta, bread, potatoes, oats, whole grains.11012
  • Avoid very low‑carb diets when trying to gain.10

You can “top off” meals to increase calories easily: cheese on casseroles, nut butter on toast, extra olive oil on vegetables, milk powder in soups or mashed potatoes.31011

4. Use high‑calorie drinks

• Drink milk, smoothies, or milkshakes between meals, not diet soda or black coffee.34911
• You can make homemade shakes with milk, nut butter, oats, yogurt, honey, and fruit.
• Weight‑gainer or protein shakes are an option if food alone isn’t enough.1212

5. Time fluids so they don’t kill your appetite

• If drinks fill you up, avoid lots of fluid right before or during meals so you can eat more.1310

6. Consider exercise (even if you mostly want fat)

Even if your goal is just “getting fat,” some strength training can:

• Help the extra calories go more toward muscle instead of only fat.34910
• Improve your appetite, making it easier to eat more.410

Basic weight training 2–3 times per week with compound movements (squats, presses, rows) is usually enough.10

7. Be gradual and monitor your body

• Aim for 0.5–1 lb per week; big jumps in weight from extreme overeating tend to be mostly fat and can harm your health.29
• Track your weight weekly and adjust calories up or down based on changes.
• If you’re underweight or have medical issues, talk to a doctor or dietitian before making big changes—they can give a target calorie range and monitor your health.3410

If you tell me:

• your age, sex, height, weight
• how active you are
• how fast you want to gain

I can help estimate how many calories you likely need per day and suggest a simple sample meal plan.