To ink over your character, in this case mini Luffy, you need to be very precise. Don't go too fast because an error is hard, or even impossible, to correct.
So take your time.
It takes 2 steps. The first step is to go over all the lines with black ink, and the second is to add some contrasts and fillings.
We'll start with the hair. You can see the work is very detailed. You need to be patient.
From here, we'll accelerate the video a bit until the second part, because the rest of this is similar to what we've already done.
Be careful not to connect the two lines for the teeth.
We'll draw some abs and some pectoral muscles.
Draw some details for the fur on his trousers.
Wait for the ink to dry, then erase the pencil markings.
Now we'll begin the second part. Draw contrasts and fillings to make the picture more life-like.
First, we'll darken the intersections, starting with the hair.
Then the ears.
Darken the pupils. For Luffy, there are no reflections in his eyes.
We'll add some details to his straw hat by drawing some short lines like this.
Next, we'll continue to darken the intersections. Same as in the first part, you need to be patient and careful about the marks you're making.
The video is slightly sped up here, too.
Draw the lines on his sandals.
To finish, you just need to darken all of his hair.