This is what you do:
1. Lift up your top eyelid by either gently pulling it up from under the brow up by lifting the top lashes.
2. Using a brush, a pencil, or a gel liner, apply eyeliner to the under edge of your lid. You are essentially filling in the base of your lash line, but instead of doing it on the top of the lid, you are doing it on the underside. Get as close to the lash line as you can.
3. When you've filled the lash line in from the underside, you might use an eye liner brush to smooth out any shaky spots in the line. I recommend making little dashes as you fill it in in Step 2. This will help keep the line even.
4. Voila! You're done! How easy is that?
Now, I don't really care for eyeliner on the bottom part of the lid, but sometimes when you close your eyes (depending on what you've used for a liner), some of the product will naturally deposit itself on your lower lashline. This is a great way to line the whole eye, and keep it light.
I highly recommend using a brush and a really dark shadow like a dark gray, a navy, or a dark brown (you can add a touch of water to get a more gel liner look) if you like the natural look like I do. Here are some other options that are great for tightlining: Bobbi Brown - Laura Mercier - or something wild for summer by Stila. Laura Mercier also makes this handy little product to transform the basic cake liner (or any powder, I'd imagine).
I found this link (and video) that does a great job of giving you a visual on what I'm talking about. Check it out!