Thank you, it’s nothing complicated, really, I drew this
on a piece of paper, then I cut it out
put it on the flask and made it hold with patafix (is that how you call that kind of gum used to fix posters and stuff on the wall?), drew its shape with a pencil, and then removed the paper thing to finally draw it with an indelible black pen. You can then remove the pencil with water when the ink is dry, if you used a good indelible pen it doesn’t leak.
And you get this :)