Inking, especially with a brush, takes a lot of practice. While you are at it try drawing too. It’s always easier to ink what you’ve drawn as it came from your own hand and mind and you know how to best handle the material. I redrew these characters from How to Draw Comics the Marvel Way and inked them up. Personally I prefer non-waterproof ink because it flows much better and that’s what I used. But there are drawbacks, especially if you plan on adding colour with physical media.