This functionality will be disabled if you do not have 2012 mailbox-level permission (Delegate envelopes).

1. Select a target mailbox which will be a delegate of yours

2. Select effective from date - it will specify a start date of the forwarding period (can be left empty to start period immediately)

3. Select effective until date - it will specify an end date of the forwarding period (can be left empty to make period indefinite)

4. Enable 'Active forwarding' toggle

5. Click the 'Save changes' button

Forwarding is configured now. If you wish to disable it at some point later, you can disable the 'Active forwarding' toggle and click the 'Save changes' button.