Document design is a more involved activity than key design. There are far more variables to consider when creating a document's schema. Some of these factors are specific to Couchbase. Others are generally applicable to document databases.
When designing a relational system, you typically start with a highly denormalized logical view of your entities. That view is then normalized into a physical model where the data is spread across several tables in an effort to minimize any possible data redundancy.
Similarly, you'll likely start your document design by creating a denormalized, logical model. With this approach, your design first considers the most complete document that your domain demands. For example, if you were building a blog, you might start with a blog document with nested posts. Within each post, there would be nested comments and tags:
{
"type": "blog",
"title": "John Zablocki'sdllHell.net",
"author": {
"name": "john.zablocki",
"email...