There's the 'engraving' method where you use a sharpish knife directtly onto the plastic and create your pattern.
There's the 'putty way' where you spread a thin layer of putty on the zimmerited area, then use a knife to engrave the pattern.
Or there's the Milliput way (epoxy putty), where you add a very very thin skin of milliput onto the zimmerited areas and use either a knife or anther tool to engrave the pattern.
Ways No.1 and No.2 are best for 'easy' patterns of zimmerit. I'd use the Miliput one on the more elaborate Zimmerit patterns, such as the 'waffle' ones... Milliput takes a little while to set and dry and it would allow that sort of pattern to be 'pushed' into it. Normal putties dry too fast for that.
I've successfully used No.1 and No.2 (I'll post pics later), but have never used No.3 (but I've seen it done).
Domi