Counting calories is really the only certain and fast way to get there. Use a calculator like this (
https://fitnessvolt.com/weight-gain-calculator/) to find out how much excess you need per day.
As for eating strategies, consume more liquid, consume more mush, consume more hyperpalatable foods. Liquid = protein shakes and smoothies which you can load up with nut butter. Mush = mashed potatoes w/tons of butter, ice cream, oatmeal, anything you don't need time to chew since eating fast keeps you from getting full too soon. Hyperpalatable = things fat people get addicted to like chips, ramen, cookies, party food, cheese, extra condiments.
t. bony bitch who's not doing these things and has spent 6 months trying to gain 5 lbs