It might be easiest in this case to just have a paper insert slipped into the bag that faces outward. This means you can print on both sides too if needed.
Yeah, you can put the instructions on the insert. I would probably have an image on the listing showing it as well (you can do a 3 or 4 step image to show directions if needed – see attached). I would also send them instructions via email to make sure they get it there as well. Otherwise, if people don’t have it put in front of them multiple ways, they may not see the instructions and complain.
If you need graphic design work, you usually have to work with a Graphic Designer. The Factory may have a “design file” that you can use, but in reality, if it is just a slip of paper, then a Graphic Designer can create that easily enough for you.
I used Mas the Sonk from our resources to get my stuff done. He’s good and quick too.