Thank you :)
You missed the Group option in custom field list. When you create a custom field you have the ability to select a Group (created before). After that, all products related this group will have the custom fields you've just created. If you don"t have a group defined, create one first in the Group section.
From next update (1.12.6), which is available now for all, you can use properties instead (or with), the custom fields. Properties are defined directly inside the product, no need to use the Groups anymore.
Properties and custom fields are quite the same at all, the big difference is that when you define custom fields you can apply it to all the products of a Group, so you can define different custom fields for different groups, while properties are product defined.
Properties are not documented for now.