Not a simple question. The removal of a hard drive depends on the Mac model. There is no generic instruction because Macs have been made since 1884 in hundreds of different form factors that require different techniques to access and remove the hard drives.
Faced with different Mac models, I would use this collection of illustrated repair guides. If the hard drive can come out, this site will show you how:
https://www.ifixit.com/Guide/Browse/Mac
An example of one of their index page for a given model:
Realistically, faced with many different models, I'd hire a student who knows about Macs and pay him a hourly rate to sit in a room and remove hard drives for a day.
As for the drives' security once removed, the most secure way to avoid the possibility of later data retrieval is to physically destroy the drive. The same student can drill about three 1/4-inch holes through the drive on a drill press. You can crush the drive with a sledge hammer. I used to work in an ammunition plant; guess how the IT guys securely disposed of old hard drives there??
Allan (Texas-born and raised)