VCF tools, define populations

[MurielGB]

Hello,

I launched the variant calling on 11 individuals using GATK and will soon have a vcf file.
I plan to calculate statistics using vcf tools for instance Fst.
However, I was wondering how I can define the population in the vcf file in order to calculate this Fst ? In orther words, defining that the ind 1, 5 and 6 are part of pop 1 and the other are part of pop 2 ?

Thanks !

Muriel