Because it's not dress-up, it's a lifestyle.
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.
Here are the details for where to get the products used in the video.
Eyeliner and spray-on bandage
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.
You can look at the full tutorial here.
Using tattoo pens
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).
You can get these pens from Amazon.