How can I provide my own Chef cookbooks?

Cookbooks are managed on a per-cloud basis, simply follow the link on a cloud's page to "Manage Cookbooks":

Here you can provide us with the details of where you're storing your cookbooks. We support Subversion and Git, e.g. our example cookbooks live on GitHub:

Once you activated custom cookbooks, you can specify which roles should execute which recipes on what events.

You do this for every role by clicking on the "Custom Recipes" tab:

Then specify the recipes that should be execute for the listed events. You can separate multiple recipes by comma.