White eyeliner can actually be useful for more than lining your eyes...in fact, I would never actually line my eyes with it. It's white, so it's a great highlighter.
These are the two main ways I would use white liner - for both, do this after you done all your other make up as the final step.
1. This is the main and most frequent use - highlight the inside corners of your eyes. It makes them look brighter and bigger. (You can also line the bottom rims of your eyes with white to make them look bigger, but this is tricky and the white doesn't blend so well on your waterline and can end up giving you a doll eye look. Great for a costume, not so great for every day.)
I draw a little v on the inside corners....
2. The second use is to make your lips look fuller. I line the outside of my lips in the cupids bow, and under the lower lip....
Then blend...you don't want to leave your face looking like a painters sketch with white lines everywhere.
For the eyes, just pat it gently until the edges are blended. For the lips do the same, being sure to blend away from your lips. If you blend the white onto your lips you'll end up losing any definition between your lip and your skin, which will make your lips look flat and smooshed, not defined and full. If you need to, redefine by reapplying lip liner.
Happy Highlighting.