To well disperse Graphene or GNPs in epoxy resin, the interface plays a key role. Surface modification will be a good choice before mixing and dispersion.

After you eventually apply surface modification for higher compatibility (as Gu Qiao Ding said), it is more suitable to disperse the nanoparticles into the epoxy resin, via e.g. ultrasonication methods. The resin is the higher viscosity environment, therefore, homogenization methods need to be applied to ensure optimum dispersion in this environment,  as issues regarding optimum dispersion appear when homogenizing nanofillers into high viscosity mixtures. Generally, the hardener, that has significantly lower viscosity compared to the resin, is added to the homogenized resin+nanofiller mixture for curing.

First you dispers the graphene nano-particles in epoxy resin using high shearing action by twin screw extruder and then add hardner to the epoxy.