Have a full meal, don't be picky about food, and try not to drink water when eating, which will occupy your stomach.
But remember, even if you want to get fat, you should keep a healthy diet and avoid eating junk food such as potato chips.
If you don't gain weight by eating like this, you need to check whether indigestion caused by deficiency and cold of the spleen and stomach makes you gain weight. And some Chinese medicines, when they are healthy and well absorbed, naturally gain weight slowly.