What in your experience is a healthy and sustainable way to bulk up from 64kg to 75kg? Not very sure where to start to learn about doing it right from the start.

I didn't try it myself, but I know people who are trying to gain weight and muscles. What they do:
They eat a lot of carbs and proteins and also work out daily. One counts his calories and has a food diary (takes pictures of his meals) to keep track of everything. The other just tries to eat as much as he can because calories counting doesn't work for everybody and is difficult if you don't cook everything yourself.