Here's how I make my Kasrkin style plastic Stormtroopers.
Clay Shaping tools (I use silicon tipped)
I start with the legs. SM Scout or Sniper Scout legs, doesn't really matter. Cut off the codpiece and the shotgun shell or whatever that is on the leg. Now there's two ways to do the shirt bottom. I prefer making one piece as shown in the picture and wrap it around to the back/butt armor. Then I push in the folds with clay tools. I used to do it with two pieces that overlap where the shirt buttons up. Lastly lightly cut the bottom seam in with an x-acto.
On to the kneepads. I start with the small ball and push it on to the right spot. Smooth to get a horizontal line over the kneecap, then flatten out the sides. Let it cure a bit and flatten/sharpen the edges with a knife.
Sharpened kneepads, and added the thigh plate. It's just a flat plate with hard edges in line with the kneepads. I like to wait until the kneepads are completely cured before I do this so I don't screw them up.You could also chop the buckle and add another plate on the right leg, but I kinda like it as is.
The gauntlets. This guy is my Sergeant so he only needs the one. For a normal guy with a rifle just do this twice. Flatten out a blob of GS (make sure the surface is smooth and wet) and cut it into the approximate shape. The point should be in line with about the middle of the hand. Next cut a little rectangle for the hand plate. Once that's cured I add a plasticard rectangle and circle for a little detail.
Last is the head. Any IG head with a visor works. You can find this specific one on the Heavy Weapons and new Sentinel Sprues. I like to cut some of the nose off so it doesn't poke through when I'm messing with the GS. I do this just like the kneepads. Work on sharpening that horizontal line, then work on the sides. Carefully add the vents with an x-acto or little plastic bit. Cut some thin rounds for the filters. I use an antenna from a vox since there's tons of them.
The bottom is the original head, my head, and the Forgeworld Respirator head. I was planning on using a few of those but they're so tiny. Why are they so small?
The only bit I didn't picture is the torso. The Scout legs have their own belt, so you have to cut the Guard's belt off. Much easier said than done.
And here he is all stuck together.
Here's his buddies painted up.
I might have missed some details. If there's any questions let me know.