Since I was 12 I wanted to gain weight but I couldn't. Recently I cracked the code and I gained 8kg in 50 days. Yes, it's like 99% fat but still a win is a win. Here's how I did at a lazy person.
Stop thinking about other stuff too much. Think more about food. Save more brain power for food. If you have other goals, consider pausing it or putting on back burner if possible for the time being while you gain momentum on gaining weight.
Less stress. It reduces appetite. Limit hard things because it reduces your will power for food. Take naps if tired. Turn on AC if hot. Don't attempt hard things if it will stress you out too much. Anything that makes you anxious - cut.
Spit out food if you feel like you will vomit. It's better than vomiting and you losing appetite.
Eat breakfast. Sleep earlier and wake up earlier. You need breakfast or else it will make the rest of the day harder.
Find what foods you like and just keep them on rotation. Save brain power from picking different food. the start is the hardest but once you find your food rotation it's easy.
Mine is just rice, beef, yoghurt, cashews, soup, and fast food once/twice a week. Find what works for you. I stopped eating eggs cos it made me want to vomit.
When you identify you are lazy, you need to adapt to gain weight. I hope this helps other undisciplined and lazy people to gain weight