Head swaps and color schemes are the best place to start.
For example pick up pile of zombie heads for Nurgle and chaos marauders for khorne.
My first
GW army was plague guard with Catachan and zombie bitz. It would be easier now with bitzsites and 3rd parties like mantic.
Then paint them in a drab imperial color like grey and add bright chaos colors like red and green. Possibly make it look like ordinary uniforms being covered in blood or slime.
I did something like that with my BLOOD!zerker army to show them falling to chaos.