It's looking good so far but I think the helmet needs a bit of work. There should be notihng below the black part of the visor, the silver part on each side ot the visor extends a bit beyond the edge of the vizor. It shouldn't have the collar/chin piece like the guard helmet base, it needs to end at the visor. The sides ote hlmet that flare out on the shoulders, needs to be shaved off and brought in close to the helmet, the Sith trooper's helmet has more of a defined corner there and the helmet doesn't extend over the shoulders at all. The visor and the top of the helmet look fine you don't need to do anything there. the armor around the but may need some sculpting work as well, it almost like a half skirt around the back. The crotch looks fine but it needs more armor on the sides of the leg and the back side. Maybe it could use a touch of drybrushing as well to bring out some of the detail on the armor. Overall it is a good custom it just needs a bit more work.
If that didn't make sense (I'm really tired, so it may be incoherent babbling
) here's apic of what needs to be fixed: