"Tightlining," or applying eyeliner to the innermost part of your eyelid, is one of the stealthiest makeup tips to ever exist.

It covers up the gap between your eyeliner and your lid...

...which helps to open up your eyes and make 'em look bigger.
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And just a touch more dramatic.
Here's what to do:
First, grab your favorite waterproof gel eyeliner.

You're going to be putting the liner on a wet part of your eyelid, which means waterproof is pretty necessary. It doesn't matter whether you use pencil liner or a liner pot plus a brush (either an eyeliner brush or a concealer brush will work!) as long as you're comfortable with the tools you're using.
FYI, gel liner is ideal for tightlining because it won't tug on your skin or bleed out of place.

And when you use gel that's also waterproof, that shit will not budge.
Looking down, pull your eyelid up and draw pencil liner onto the upper waterline.
This part is going to take a LOT of practice. Do your best to get into a ~zen~ zone and try to avoid blinking while you're applying liner.
Or, do the same thing with gel eyeliner on a brush, but instead of drawing on the liner with a pencil, gently "push" the liner into your lash line.
Do NOT push the brush into your eyeball or use excessive force. All you need to do is press the brush carefully against your lash line so that it transfers eyeliner onto the skin.
If you want even MORE eyeliner, release your hand from your eyelid and press another round of liner onto the top of your lash line.
This step is optional, but it'll help to make your eyes look even bigger (if that's what you're going for).
Then, finish off with some mascara...
...and BAM! You have some ~next level~ eye makeup.

Watch the entire tutorial from Wayne Goss here.