Good question. I used LGD-4033 (Ligandrol) to increase my appetite, and it helped a lot.
Read this article on going from
skinny to buff. It has a lot of good advice, primarily in regards to eating.
Use a lot of shakes, man. I like Rich Piana's Mutant Mass, but ON has a decent one, too. Get a blender, it will be your best friend.
When I was bulking up for my photoshoot, I was eating like 4,000 calories a day (and I was 5'8" and 170 pounds). That's a lot of freaking calories!
Shakes will always be your best friend. Mix oatmeal, milk, whey protein, bananas, and nut butter (almond or peanut are your best bet) for a huge boost of calories.