You must have the option activated to have access to the following information.
First of all, you will need to setup tasks to trigger purchases. To do so, you can access them by opening a workflow. Workflows are accessible through Setup > Processor > Workflow in the menu or from Scheduling > Maintenance > Workflow.
You can now configure one of the tasks to trigger the purchases for the Just in time. You can do so either in the Workflow management detail table or directly in a task.
Only one task can be configured per workflow. Changing it will affect the dates for the purchases.
Once this is done, adjust the Order security delay to trigger the purchases correctly. You can access it through Scheduling > Maintenance > Configuration