With great difficulty.
So, you start off with something like a complimentary RNA sequence. Something like CRISPR but instead of just chopping up the SLC24A5 gene at the target region it activates some other mechanism that does something nasty like produce a neurotoxin.
You can throw in some tech tech tech words around that but something so exact that can activate an arbitrary biochemical mechanism doesn't really exist currently so can't really be described
So lets talk effects.
The variant in question is rs1426654. It's responsible for a large fraction of the difference in skin pigmentation between western Europeans and other groups.
It's present in
99.6% of european people.
23.8% of native african people.
84% of south asian people.
99% of Ashkenazi Jewish people.
So it's not going to be very exact because the variant is not unique to europeans, if it kills people then it's going to be killing half the human population of the planet.
Any such mechanism might also be fragile, one mutation in the virus and suddenly it's targeting everyone else as well.