Step 1:
Go to Campaigns > Create Campaign.

Step 2:
Select Email or SMS Campaign and click on “Create Campaign »” button.

Step 3:
Name your campaign, select the segment you'd want to have it sent to and click on “Continue to Content” button.

Step 4:
Set a Subject line, create your email, then click on “Continue to Review” button.

Step 5:
Next, review your campaign and Klaviyo's suggestions and schedule or send the campaign when you're ready.

This Klaviyo article offers more details on how to set up your campaign.

Here's a quick video tutorial for more details: