How to enable iOS app for iCloud Documents

1. a) New app: create a new App ID in Member Center on Apple Developer website (https://developer.apple.com/). The account must have Agent or Admin role.

- open Certificates, Identifiers & Profiles, select Identifiers

- click the + sign to create a new App ID.
- App ID Description: enter a Name, for example - iCloudDriveExplorer
- App ID Prefix: it defaults to the Team ID and is not editable
- App ID Suffix: select the Explicit App ID option - it is a must for using iCloud. Example: net.samples.iCloudDriverExplorer
- App Services: check the iCloud option and select either Compatible with Xcode 5 or Include CloudKit support (requires Xcode 6), whichever suits the needs. Note: the status initially will be set to Configurable with a yellow indicator - that is OK.

- click Continue and complete the App ID creation process.

1. b) Existing app: Edit the App ID
- check the iCloud box option and select either Compatible with Xcode 5 or Include CloudKit support (requires Xcode 6), whichever suits the needs. Note: the status initially will be set to Configurable with a yellow indicator - that is OK.

2. In Xcode - create a new project or configure an existing project to enable iCloud Document entitlement.
- select the project's target and open the Capabilities tab.

- expand the iCloud row and switch the iCloud ON. Xcode will create the project entitlement plist file, in this example named: iCloudDriveExplorer.entitlements

The contents of the project entitlements file will look similar to this:

<plist version="1.0">
<dict>
    <key>com.apple.developer.icloud-container-identifiers</key>
    <array/>
    <key>com.apple.developer.ubiquity-kvstore-identifier</key>
    <string>$(TeamIdentifierPrefix)$(CFBundleIdentifier)</string>
</dict>
</plist>

- check required iCloud services: Key-value storage, iCloud Documents and CloudKit, whatever is needed.

When enabling iCloud Documents, Xcode will offer to use either the default container or custom containers. Configuring custom containers is a subject for another post.
For the default container Xcode will add a container entitlement to the project entitlments file and will update the Provisioning Profile. After this step, the status indicator for iCloud in the Member Center will become green:

After Xcode adds containers to the project entitlement file, it will be similar to this:

<plist version="1.0">
<dict>
    <key>com.apple.developer.icloud-container-identifiers</key>
    <array>
        <string>iCloud.$(CFBundleIdentifier)</string>
    </array>
    <key>com.apple.developer.icloud-services</key>
    <array>
        <string>CloudDocuments</string>
    </array>
    <key>com.apple.developer.ubiquity-container-identifiers</key>
    <array>
        <string>iCloud.$(CFBundleIdentifier)</string>
    </array>
    <key>com.apple.developer.ubiquity-kvstore-identifier</key>
    <string>$(TeamIdentifierPrefix)$(CFBundleIdentifier)</string>
</dict>
</plist>

Important:
Enabling iCloud for an app requires an Xcode Developer Account with Agent or Admin role.
Even though Xcode allows to have multiple Developer Accounts (Xcode > Preferences > Accounts) and prompts to choose the account with which to enable iCloud, it may fail to create a container entitlement:
Add the "iCloud containers" entitlement to your App ID.

In this case Xcode will offer the Fix it option. However, running the Fix will not prompt for the Developer Account and may fail if the account Xcode choses to run with does not have Agent or Admin role.
One workaround is to remove, temporarily, other accounts from Xcode and only leave the Admin (or Agent) account. The other accounts can be exported into a file (Xcode > Preferences > Accounts > select Apple ID then click the Setting icon on the bottom left  > Export Developer Accounts).
When iCloud configuration complete, these accounts can be easily imported back.

Update:
In later versions of Xcode, for example 7.2, it is also possible to create and enable iCloud entitlements entirely from within Xcode. As long as the Developer Account has Agent or Admin role, Xcode will create App ID automatically. It will also modify the Provisioning Profile to enable iCloud service when the iCloud capability is switched on. And it will create entitlements for the default iCloud container. Manually creating App ID and enabling it for iCloud via Member Center is no longer the only option.