The pens have a circular nib and have to be held approximately vertically.
Where there is a change of direction of the line it is difficult to create a sharp corner.
I have added a drop shadow to improve the illusion of depth though this would not, of course, be found on a modelled example.
Regrettably the sketch is not as symmetrical as it should be, but it does give an indication of how a basic framework may be readily developed into a solid design with the suggestion of a more complex character in its arrangement.
You can see there is some blotting at the beginning of lines – look at the red lines top right – as well as a little unevenness in line thickness.