Ian, as no-one has responded, I'll give my view.
The concept is simple and but you will need agreement from all owners before you start, as the deed forms part of your home deed when you come to sell.
I would go to a lawyer or Asesoria to get the ball rolling. There are 'standard' terms that can be used as a starter and they can then be registered with the local Notary.
Someone has to decide the percentage responsibilty that each apartment takes. This is easy if they are all the same size, but may get messy if some are larger units than others, and some have made 'illegal' additions.
After this is agreed, registered and setup, then you are away! Vote in a President and do whatever the meeting majority agrees. And then pay and percentage based on your 'share' percentage.