2. Hold your brush perpendicularly to the surface
No matter what kind of linework you do, always hold your brush perpendicular to the skin.
3. Get the right paint consistency
Getting the right paint consistency will help you obtain vibrant colors and sharp lines. Creamy and inky paint consistencies are what you want for linework.
Use creamy consistency for your white lines and inky (slightly more watery than creamy) consistency for your black and colored lines.