This card by @DanRodriguez got me thinking about how cosplayers make tattoos for their characters. I assume they're not all getting tattooed for the sake of one cosplay! So I did some investigating, and compiled my favorite techniques for creating a temporary tattoo.
Transfer Paper and Adhesive
Commander Holly is an amazing cosplayer, and their tutorials are extremely helpful. This technique is great because the tattoos look very natural, and the effect is long-lasting. You're very unlikely to sweat this tattoo off during a con (you know how hot it can get!). The products are a little bit expensive though, so if you think you won't be doing this often it might not be worth it.
This is almost the inexpensive version of transfer paper and adhesive. This tattoo will be waterproof and last for about three days, so you won't need to worry about smudging or sweating with this one either. It's not as precise as printing out your own design, so it might not be great for a really detailed image, and color would be difficult, but if you need something cheap, simple, and durable this might be the best choice for you.
It's really not a good idea to use Sharpies or acrylic paints on your skin, since they're often full of toxins. Instead, nontoxic tattoo markers are a great alternative! These are perfect if you want color, or would prefer to draw freehand instead of using a design (though the video above has a template).
This card by @DaniaChicago will walk you through a great technique. There are products on the market that are intended to be used as concealer for tattoos, but they're often very expensive. This technique will be much cheaper and works just as well!