Eating as much as you can through the day, have that all important bedtime snack that's very high in saturated fat
The other thing, though some might not want to try them, is weight gain shakes, if you are generally sedentary they can help you really pile on serious weight in addition to a very fattening diet, one big tub I got in 2010 helped me boost my at the time 47" waist to 53.5" in about 7 to 8 weeks.
Eat lots of desserts, plenty fried foods, and don't resist the takeaway on the way home.
If your metabolism is fast you will find it hard to gain weight fast.