I put on 28 pounds in 4 months when I started strength training. For me, I was underweight at 6'4 - 175, and I wanted to fill out my frame.
I got a good workout plan that worked on my big lifts, the squat, deadlift, bench, and overhead press.
As for the diet, if you're serious, you're just going to honestly need to focus on eating way more than you're used to. Don't worry about snacking 'keeping your metabolism up'. You should be eating to blow away your metabolism. I ate a meal every three hours, and was drinking about a gallon of homo milk every two days. I'm also lactose intolerant, but at a level that I could manage by spreading my milk intake out in small amounts throughout the day, and then just sucking up the occasional gas and discomfort if I overdid it.
The bottom line is that if you struggle with putting on weight, it's going to take a serious mental shift to make any real difference. There are very few quick and easy solutions to a lifestyle change.