1. First step of all, as always, apply basecoat.
2. Apply the light coloured polish - in this case it's nudish pink. Always apply the light colours first to contrast with the darker colours. This time I chose a colour that's very close to my natural nail colours so that in a glimpse it'd give the effect that my red polish had been smeared off in the middle.
3. After the light layer is reasonably dry, apply the dark layer. First, apply the part that's wider at the bottom and polish diagonally all the way to the edge of your nail. It takes focus and accuracy to run the brush straight, so take your time. Avoid metallic textured colours that show your brush strokes; choose plain or finely glittered [dusty glitter kind]. With these you can polish slowly and carefully with smooth result. Also, don't use too much polish on the brush - just use a little to create the diagonal line. You can always fill the rest later if it ends up being too transparent.
4. Using the dark diagonal layer as guide, apply the other dark layer side, running the brush parallel with the slant of the first dark layer side. As in the first layer, take the time to create straight lines first, then filling the rest afterwards.
5. Let dry. If possible, use only 1 coat of dark layer, but most likely the nail surface would still look uneven - the dark layer would naturally look raised. So take the light coloured polish again, and run lightly once between the dark layers - just to even out the surface.
6. Apply top coat.
It's really not that difficult!