About スミ入れ (Sumi-ire: lining black?)
It may be better to prepare two types of Gundam-malker(BANDAI pens), black and gray.
Using a black-malker for the part you want to emphasize(Only Line around chin portion of the Gundam's face) and a gray-malker for the other parts(Other line of face parts)
I prefer gray-malker because It is over-emphasized when using a black-malker as a whole.
This HP may be helpful for painting Gunpla.https://hobby.dengeki.com/ganpla_howto/
The HP explains about detail-up of straight built Ganpla
*素組み(Sugumi: straight build)
Sry, Please use the translation app because it is all written in Japanese