How do I pre-pay for my AWS Elastic Beanstalk's EC2 instances?

I understand that I can reduce my AWS hosting costs by using "reserved EC2 instances", and the process of acquiring reserved instances seems straightforward enough. However, it isn't clear to me how to use these reserved instances in my existing Elastic Beanstalk applications.

Once I have reserved instances, will they automatically be used (in place of pay-as-you-go instances)? Is there something I need to do do "switch" my existing EB applications over to use reserved instances? Will new instances stared (e.g., by auto-scaled) use reserved instances?

How do ensure that my AWS Elastic Beanstalk uses pre-paid, reserved EC2 instances?