You buy what ever share you wish to buy, the housing association / builder will then buy the remaining share and you will rent the remaining share from them at a pre-agreed rate. You have the option to buy out this remaining share at any time but you will be buying the remaining percentage of the property value at the value that is placed on it when you buy out the remainder and not the price you paid for it today.
Buy a £100,000 house and take an 85% (£85,000) share. Co-Ownership own 15% (£15,000)
You then put down a deposit of what ever you need on the balance you owe (say 10% (£8,500) and then you take out a mortgage on the remaining balance (£85,000 - £8,500 = £76,500).
Co-Ownership will purchase the remainder and you will pay a rental based on their portion of the property. It may be an annual charge of around 2.5% and you will then pay Co-Ownership this sum divied by 12 months as your rental for the portion of the property that they own. (£15,000 x 2.5% = £375 / 12 months = £31.25 per month rental.
If you then decided to buy out the remaining 15% portion of the property then Co-Ownership will gwet the property revalued and you will then buy out the property based on 15% of the new valuation. So if the house has doubled in value since you bought it and is worth £200,000 then Co-Ownership would require 15% of this figure as the buyout price (£200,000 x 15% = £30,000)
Hope that all makes sense for you.