From the left navigation menu, click on your Team

Click on the 3-dot menu icon for the Objective you wish to add a Key Result

Select Add Result

Add the Key Result Name

Add a Description

Search and select the person who will be the Lead

Select how the Key Result will be measured

Add Start value and Target value

Select Key Result as Result Type

In Advanced Options, you can add Contributors. If you know this Key Result won't start to develop until you've worked on Initiatives that'll affect it, you can add a Start Date. You can also add a Due Date.

Click Save