First start out with clean nails.
If you want you can buff them out to make them smooth, or manicure your nails before you begin.(It makes it look cleaner.)

Next carefully pour a little of the red nail polish on the index card. Take your make-up sponge and dab it in the polish. Gently stroke upwards from the tip towards the cuticle.(It's okay if it looks a little light.)

Repeat step 2 with the black nail polish, except instead of strokes dab in an upward motion.(It's alright if it feels a little rough, that is what gives it a cooler effect.) If it becomes too dark then on the index card, pour both colors and mix to desired color.

Lastly put clear coat after the paint dries. And then you are done. Repeat steps 1-4 on the other hand.