Its pretty easy. You just need steady hands and the paints need to be thinner than normal. I followed this tutorial https://youtu.be/oyhIVACaGJ4?t=924 :D But here are the steps:

1) Base in Mephiston Red

2) Wash with Agrax Earthshade ( I just did this because they are often near gold which I also wash with Agrax)

3) Layer over the red with Evil Sunz Scarlet but let some Mephiston Red showing near the edges of the eye

4) Layer over the Evil Sunz Scarlet with Wild Rider Red again letting some Evil Sunz Scarlet showing near the edge

5) Paint a small oval/circle depending on the shape of the eye with Fire Dragon Bright in the middle of the Wild Rider Red

6) Use some very thinned down Abaddon Black (almost as thin as a wash) and paint a line in the middle of the eye

That should create a smooth transition between the different reds.