I'm in the same boat as you. I'm not a twig, but I do wanna gain weight (right now I'm about 6'2 190lbs and I'd like to be 220 then slim to maybe 210 for the summer).
First off, eat A LOT. Don't stuff with crappy junk food tho - lots of chicken, skim milk, lean meat. Get extra calories.
Next, be dedicated to working out. Lift heavy with lower reps than you usually do - I find that a good weight is such that you can do 4 reps yourself but need help before or during the 6th - (
Max OT). Stick to the most basic, compound movements like bench press, dead lift, and squats.
Weight gain supplements are pretty nice too, just get something with lots of calories and protein and drink a couple a day. I really like
N-Large 2. It has 52g of protein per serving and a nice 620 calories.
Good luck!