You can use the Apprentice rules almost directly. Instead of
having two apprentice classes, you just have 1. And your HD starts off at d4.
Doing so misses the real fun behind 0-level characters... total customization of your character. There were rules in the old Greyhawk Adventures book (2nd ed) for 0-level characters that worked well. Here's the basic concept:
Instead of XP, you earn AP (advancement points? apprentice points?). With AP, you can buy something to improve your character from their base of a 0-level commoner. Want a d6 HD instead of d4? That'll be some AP. Want d12 HD? That'll be a lot more. Want 2 skill points per level or 8? Want access to the wizard/cleric/bard/paladin/ranger spell list? Bonus feats? Class-specific abilities? You want that 18 strength? Everything has an AP value that you purchase.
The tricky part is ensuring everything has a proper cost, and
there's no obvious abuses of the system. It's not easy, but with 0-level characters, you're kind of off the beaten path anyways. Some mild imbalances are bound to occur.
Once you reach 1000 AP, you've hit level 1. Happy hunting.