I've had to gain weight many a time. For me the key is looking for calorie dense food, so low volume high calorie.
It does depend a bit on the type of food you enjoy. It's a lot easier to gain weight eating the stuff you enjoy than forcing down food you don't.
The usual meal plan for weight gain is 3 meals and 3 snacks a day, but obviously you can Taylor to suit your needs.
Nuts and nut butters are good, ground nuts can be added to all manor of curries, sweet and savoury dishes. Cheese is your friend add to any pasta or potato dish, make cheese sauce for veg.
Speaking of veg, roasting will increase calories but you can also add butter to steamed veg, or oil based dressings to salad. Mashed potato can be hugely calorific if mashed with cream, cheese and butter you can also add egg yolk.
Make scrambled eggs with extra yolks and plenty of butter.
If eating sandwiches look for things that have a mayonnaise base, cheese or you can double up on filling toasties are good for this - ham and cheese, Brie and bacon, or cold you could do egg and avocado or Pita with houmous and falafel, bagel smoked salmon and cream cheese.
Also think about what your are drinking, milk based drinks are good.