Markers
So there you have it!
Mark to your heart's content!
It is a good idea to plan your work session so that you do all your decorating at once. Do not let ink dry in the tip of the pen. The smaller tip sizes are not very tolerant of being left unused for very long (even 5 minutes). The larger ones are a little more forgiving.
It is best if you clean the tip of the pen after every work session. They are really easy to clean. Let's clean one now...