Just go to booty bay and click on one of the plagued boxes and you're infected. Ride to where you want to turn into a zombie (Zul'Gurub is nice for level 50 Zombies)

The NPC zombies seem to cap at level 50, even if you kill higher level mobs... so I guess you can't get a super army of level 70 elite zombies going, at least in STV...

And don't walk near the Argent Dawn agents all over the cities before you actually turn into a zombie, they're there to cure you.