There are lots of different ways to panel line kit, but I would say the default beginner way is to use the thin Gundam Markers specifically designed for the task. They come in 3 colors brown, black and grey. I would point you away from using pure black unless you're lining grey or another dark color, though, as it makes the lines stand out too much for my taste. It's all personal preference in the end. I buy mine from HLJ, but with cheap shipping it'll take a while to get to you.
They markers come in two types. The
traditional small tip ones that act like normal markers.
Then there are the new
pour type. With these, you just tap them on the line and capillary action fills in the rest. They leave a little dot where you tapped, so you have to wipe the excess with a Q-Tip or something. These are my personal favorite, because they provide a really consistent finish across the entire line.