This will not work if your brows are too arched (so if they are, let them grow out and get it reshaped).
For those who would prefer a step by step tutorial, here's one! I have prepared an image, filling the brows in on photoshop.
*Base brow image credits to princess-livia.com. Editings to the image were done by myself.1. Start with a clean brow.
2. Draw a straight line from the bottom-middle of your brow to the tail of your brow.
3. From the top-middle (highest point of your brow), draw a straight line to the front of your brow.
4. Fill in the rest of your brows, gradiating it to make sure it is lighter and softer in front.
Try not to use a shade that's too dark to avoid the brows looking too harsh. Your brow colour should be up to 2 shades darker if you have light coloured hair, and 2 shades lighter if you have dark coloured hair. If you have lighter hair colour but very dark brow hairs like me, use a colour corrector or brow mascara to tint your brows :).
And voila! Here are your straight korean ulzzang brows! Fuller looking straight brows give the impression of a more youthful look. Let me know how you guys like this! ^^