Skinny to buff
I am in the same boat. I have been eating often and using heavier weights with low reps .ag the gym with little cardio.
Eat alot of good foods oatmeal,brown rice, salmon, tuna. High carb foods and rich in protien. Do smaller meals 6 meals a day not 3 big meals and not junk foods but nutrient food so your body uses the content for repair and growth. Do low rep routines 6/8 but try increase weight the weeks that follow. Also have a good source of protien maybe eggs/milk/honey or a protien shake 30 mins before bed. The body grows and repairs in our sleep so protiens are good to have in your system. Have at least two days off a week from training.
Try wearing a weight vest when doing your workout.lift heavy weights but not to heavy small reps.