[Help] Need advices for a portrait

Hi there,

It's my first attempt of sculpting the portrait of a real person. It's Hyunjin from Stray Kidz.

But I don't know what to do to make it looks alike. I tried the grid method but it isn't good for global shape. I can use the grid for details like eyes, lips, placement. But for the global shape of the head... I don't know how to make it similar