I find meal prep is the best way to gain weight. Use cronometer.com or another nutrition tracker to design a week of balanced, high calorie meals, make them in batches, then just eat then on schedule. You can add snacks for extra calories when you're not too full.

Shakes, burritos, rice and beans, and grain bowls are all good formats for packing in calories. For snacks, peanut butter, nuts, seeds, and trail mix are a good source of fats.