To add Key Results to your Objectives:
From the left navigation menu, click on your Group
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 Settings, 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.
Note: You can also add Key Results from My profile and on the Objective details page.