Step 1:
Go to Flows > Create Flow

Step 2:
Select “Create from Scratch”.

Step 3:
Name your flow and click on “Create Flow”.

Step 4:
You are sent to the edit page of your recently created flow. Here you can choose the trigger for the flow. Choose “Segment” from the left sidebar option menu.

Step 5:
Choose your previously created segment from the dropdown list to trigger the flow and click on “Done”.

If you'd like customers to exit the flow if they place another order, add a Flow Filter:

Select "What someone has done or not done" > "Placed order" > "zero times" > "since starting this flow" to ensure only customers that are not placing another order while being in this flow would keep receiving these emails:

Step 6:
Configure the flow with the actions you want to do for your customers. More on how to create a Klaviyo flow can be found in this Klaviyo article.
Once the flow is ready, set it live from the "Review and Turn On" button.

Note:
Use the “Back-Populate Flow Recipients” option if you wish to add to the flow the customers who were already added to the segment before creating the flow. More on this subject in this article from Klaviyo.

Here is a quick video tutorial that walks you through all the above steps: