first you wanna spawn your zombie, preferably one of the civilians or maybe a combine zombie (for the funky noises)

then you wanna spawn your death trooper ragdoll

and now you wanna take out your advanced bonemerge tool (from the mod of the same name) and bonemerge the death trooper to the zombie.

now you wanna get out your material tool (it HAS to be the vanilla material tool, trust me) and select an invisible material (id recommend using the invisible material mod) and shoot the zombie with your toolgun.

now it looks like its JUST the death trooper model! and then you just use your duplicator tool and save your new undead "friend" as a dupe.

also the reason you gotta use the vanilla material tool is because materials applied by the submaterial tool don't stick around on death, meaning the base zombie you used will reappear after you've killed the death trooper, and look super fugly.